Semiconductor photo-detector, semiconductor photodetection device, and production methods thereof

ABSTRACT

In a semiconductor photo-detector of the present invention, a first semiconductor layer, a second semiconductor layer having, and a photo-absorption part composed of a photo-absorption layer sandwiched between these layers are disposed on a substrate, at least the photo-absorption layer is formed at a position apart inwardly by a finite length from an end surface of the substrate, an end surface of the second semiconductor layer and the substrate or the end surface of the substrate is provided with a light incident facet angled inwardly as it separates from the surface of the second semiconductor or the surface of the substrate. Further, a groove as a guide of an optical waveguide for guiding incident light is disposed opposing the light incident facet, or the substrate end surface at the light incident facet side is protruded by a finite length from a tip part of the light incident facet, or between the optical waveguide and the semiconductor photo-detector is buried in a solid or liquid, or a main reaching area of incident light refracted at an upper layer of the photo-absorption layer is terminated with a substance having a smaller refractive index than the semiconductor layer of photo-absorption region part, or the light incident facet and its vicinity are buried in an organic substance.

CROSS RBFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 09/184,218,filed Nov. 2, 1998, now U.S. Pat. No. 6,353,250, in the name of HidekiFukano and entitled SEMICONDUCTOR PHOTO-DETECTOR, SEMICONDUCTORPHOTODETECTION DEVICE, AND PRODUCTION METHODS THEREOF.

This application is based on Japanese Patent Application Nos. 9-305148(1997) filed Nov. 7, 1997, 9-305149 (1997) filed Nov. 7, 1997, 9-332587(1997) filed Dec. 3, 1997, 10-98 (1998) filed Jan. 5, 1998, 10-1466(1998) filed Jan. 7, 1998, 10-8236 filed Jan. 20, 1998, 10-192793 (1998)filed Jul. 8, 1998, the contents of which are incorporated hereinto byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a refraction type semiconductorphoto-detector, a semiconductor photo-detection device comprising thesemiconductor photo-detector combined with an optical waveguide, andproduction methods thereof.

2. Description of the Related Art

Conventional refraction type semiconductor photo-detectors have astructure in which, as shown in FIG. 1A, a light incident angled facet21 a is formed across a photo-absorption layer 23 a from the topsurface, or, as shown in FIG. 1B, an angle begins from a layer rightbeneath a photo-absorption layer 23 b while contacting with thephoto-absorption layer 23 b (for example, Japanese Patent Application9-52760 (1997)).

In FIG. 1A, numeral 21 a indicates a light incident facet, 22 a is ap-InP layer, 23 a is an InGaAs photo-absorption layer, 24 a is an n-InPlayer, 25 a is an n-InP layer, 26 a is a p electrode, and 27 a is an nelectrode.

Further, in FIG. 1B, numeral 21 b indicates a light incident facet, 22 bis a p-InP layer, 23 b is an InGaAs photo-absorption layer, 24 b is ann-InP layer, 25 b is an n-InP substrate, 26 b is a p electrode, and 27 bis an n-electrode.

Still further, in FIGS. 1A and 1B, numeral 28 indicates an optical fiberas an example of an optical waveguide for conducting the incident light.This optical waveguide 28 is combined with a semiconductorphoto-detector shown in FIG. 1A or 1B to construct a semiconductorphoto-detection device.

In the production process of above described photo-detector, whenforming the reverse-mesa optical incident facet by using wet etchingwith bromine-methanol or the like, in etching including thephoto-absorption layer 23 a as a narrow gap or where thephoto-absorption layer 23 a exists close to the etching, thephoto-absorption layer 23 a as a narrow-gap is relatively fast inetching speed and, since side etching is liable to occur, an etchingirregularity such as uneven side etching tends to generate during deepetching, resulting in a problem of generating fine irregularities orwaves on the etching surface.

When the spot size of incident light is large, effect of irregularitiesor waves is small. However, when an optical beam is focused and appliedusing a tapered fiber or a lens, this effect becomes conspicuous, thebeam is diffused, and focusing of the beam is degraded.

Further, in the prior art structure, in order to obtain a high-speedresponse, the incident position must be set at the top surface side aspossible so that the photo-absorption area is the smallest, when theincident light position is moved down to the substrate side, thephoto-absorption part is required to be increased in length to makephoto-absorption possible.

As a result, the photo-absorption area is increased resulting indegraded high-speed response characteristics.

Still further, in the above-described prior art semiconductorphoto-detector, the chip is formed by using cleavage or the like fromthe vicinity of the light incident angled facet.

Thus, the chip does not have a guide structure for optical-connectionwith an optical waveguide such as an optical fiber.

At the time to connect the optical fiber with the photo-detectoroptically, when the optical beam center of the optical fiber is fittedto the center of the optical-absorption area of the photo-detector, theresponsivity becomes maximum, and when the optical beam center of theoptical fiber is shifted from the center of the optical-absorption areaof the photo-detector, the optical-absorption amount of thephoto-detector is decreased, thereby the responsivity is deteriorated.

Although permissible range of the shifting depends on size of thephoto-detector and a direction of the shifting, etc., the permissiblerange is usually several μm in the minimum direction.

As a result, in optical coupling with fibers or the like, finemechanical adjustment of fiber optical end with an accuracy of severalmicrons is required to a position where the responsivity is the maximum.

Therefore, there is a problem in that when fabricating a module(semiconductor photo-detection device) by combining a photo-detectorwith a fiber, a very precise positioning technique is necessary, andeven a small deviation generates a degraded responsivity or a degradedresponse speed.

Therefore, in general, one or two lenses are inserted between thephoto-detector and the fiber to moderate the positioning accuracy.

However, there is a problem in that the insertion of such a lens systemleads to increases in the number of parts or fabrication steps resultingin an increase in module cost.

Further, there is a report of a structure in which to perform goodoptical coupling with the fiber without using the above lens system, thephoto-detector is mounted on an optical fiber holding substrate having aV-shaped groove comprising silicon or the like. However, in thisconstruction, it is required that the optical fiber holding substrateand the photo-detector be connected in high mechanical precision, whichrequires a high-precision positioning technique, and a small deviationgenerates reduction of responsivity or response speed.

Still further, even when the lens system is inserted as described above,there is some distance deviation between the device and the lens systemduring positioning, which is a cause of deviation in responsivity in adevice with a small misalignment tolerance.

Yet further, in the above-described prior art semiconductorphoto-detector, the electrodes 26 a and 26 b of the upper layer are, ingeneral, alloyed with the semiconductor layer by heat treating metalssuch as AuZnNi for the case of p-type or AuGeNi for the case of n-typeto form ohmic electrodes.

By virtue of such alloying, fine irregularities are generated betweenthe electrodes 26 a and 26 b and the p-InP layers 22 a and 22 b assemiconductor, even if refracted light reaches here, it is diffusereflected or absorbed by the electrode metal itself, and the electrodepart is small in light reflectivity.

Therefore, although the thicknesses of the photo-absorption layers 23 aand 23 b can be reduced by increasing the effective absorption lengththrough which the refracted light transits diagonally with respect tothe layer thickness direction which is a characteristic of therefraction type semiconductor photo-detector, to obtain a sufficientlylarge responsivity, refracted light to the photo-absorption layers 23 aand 23 b is required to be sufficiently absorbed by one transit, therehas been a limitation in reducing the thickness of the photo-absorptionlayers 23 a and 23 b.

As a result, a transit time of carriers transitting the photo-absorptionlayers 23 a and 23 b is a limitation factor of response speed of thesemiconductor photo-detector, and there is a problem in that anultra-high speed and high responsivity device cannot be fabricated.

Yet further, in the prior art refraction type semiconductorphoto-detection device, for example, as shown in FIG. 1A, the lightincident facet 21 a of the refraction type semiconductor photo-detectorand an optical waveguide such as a single mode optical fiber aredisposed in opposition, and a gas such as air or an inert gas is filledin between.

Here, since the gas has a refractive index of nearly 1 and therefractive index of the photo-detector material is constant, therefraction angle at the light incident facet 21 a is determined only bythe reverse-mesa angle.

In general, in the production process of refraction type semiconductorphoto-detector, when fabrication is made by determining the reverse-mesaangle, mesa angles of devices in the same wafer become all in line witheach other.

Since the refraction type semiconductor photo-detector utilizes anincrease in effective absorption length by transiting light the opticalabsorption layer by refraction, the refraction angle is determinedequally in the prior art, and the effective absorption length is alsoconstant.

Therefore, there is a problem in that to change the effective absorptionlength according to various applications, wafers of different mesaangles or different absorption layer thicknesses are required to beprepared to change the refraction angle.

SUMMARY OF THE INVENTION

A first object of the present invention is, in a refraction typephoto-detector, to provide a refraction type semiconductorphoto-detector and a production method thereof which has a very flatangled light incident facet even to a fine size light beam.

A second object of the present invention is, in a refraction typephoto-detector, to provide a semiconductor photo-detector, asemiconductor photo-detection device and production methods thereofwhich are capable of easily making high precision optical coupling whenmaking optical coupling with fibers.

Further, a third object of the present invention is, in a refractiontype photo-detector, to provide a semiconductor photo-detector which canprovide a high responsivity even with a thin optical absorption layerand is capable of making ultrahigh-speed operation.

Still further, a fourth object of the present invention is, in asemiconductor photo-detection device comprising a refraction typesemiconductor photo-detector and an optical waveguide disposed inopposition to the photo-detector, to provide a semiconductorphoto-detection device which is capable of making adjustment ofresponsivity according to the application using refraction typesemiconductor photo-detectors of the same layer structure and the samemesa angles.

A semiconductor photo-detector according to the present invention whichattains the above object is characterized in that: a first semiconductorlayer having a first conduction type, a second semiconductor layerhaving a second conduction type, and a photo-absorption part comprisinga photo-absorption layer sandwiched between the first semiconductorlayer and the second semiconductor layer are disposed on a substrate: atleast the photo-absorption layer is formed at a position apart inside bya finite length from an end surface of the substrate; the end surface ofthe second semiconductor layer and the substrate or the end surface ofthe substrate is provided with a light incident facet angled inwardly asit separates from the surface of the second semiconductor or the surfaceof the substrate; and light incident to the light incident facet isrefracted at the light incident facet and transits the photo-absorptionlayer diagonally with respect to the layer thickness direction.

Further, a production method of the semiconductor photo-detectoraccording to the present invention which attains the above object ischaracterized in that: a first semiconductor layer having an intrinsicor a first conduction type, a second semiconductor layer having the samefirst conduction type, and a growth layer comprising a photo-absorptionpart including a photo-absorption layer sandwiched between the firstsemiconductor layer and the second semiconductor layer are disposed on asubstrate; a main inside part of the first semiconductor layer at thesurface side, or the inside part and part of photo-absorption layer isconverted selectively to a second conduction type by diffusion of animpurity; and an end surface of the substrate side growth layer exceptfor the photo-absorption layer or the substrate is provided with a lightincident facet angled inwardly as it separates from the surface sidefrom a position apart by a finite length in a direction parallel to thesubstrate surface with respect to the photo-absorption part comprisingthe photo-absorption layer, whereby obtaining a semiconductorphoto-detector in which incident light is refracted at the lightincident facet and transits the photo-absorption layer diagonally withrespect to the layer thickness direction.

Still further, a production method of the semiconductor photo-detectoraccording to the present invention which attains the above object ischaracterized in that: a first semiconductor layer having an intrinsicor a first conduction type, a second semiconductor layer having the samefirst conduction type, and a growth layer comprising a photo-absorptionpart including a photo-absorption layer sandwiched between the firstsemiconductor layer and the second semiconductor layer are disposed on asubstrate; a main inside part of the first semiconductor layer at thesurface side, or the inside part and part of photo-absorption layer isconverted selectively to a second conduction type by ion implantationand subsequent anneal; an end surface of the substrate side growth layerexcept for the photo-absorption layer or the substrate is provided witha light incident facet angled inwardly as it separates from the surfaceside from a position apart by a finite length in a direction parallel tothe substrate surface with respect to the photo-absorption partcomprising the photo-absorption layer, whereby obtaining a semiconductorphoto-detector in which incident light is refracted at the lightincident facet and transits the photo-absorption layer diagonally withrespect to the layer thickness direction.

Yet further, a semiconductor photo-detector according to the presentinvention which attains the above object is characterized in that: afirst conduction type semiconductor layer, a photo-absorption layercomprising an intrinsic or a first conduction type semiconductor layer,or a superlattice semiconductor layer or a multiple quantum wellsemiconductor layer, and a schottky electrode are disposed on asubstrate; a semiconductor multilayered structure of largeschottky-barrier height having a schottky barrier higher in schottkybarrier height than the schottky barrier between the photo-absorptionlayer and the schottky electrode is formed between the photo-absorptionlayer and the schottky electrode; and an end surface of the substrateside growth layer except for the photo-absorption layer or the substrateis provided with a light incident facet angled inwardly as it separatesfrom the surface side from a position apart by a finite length in adirection parallel to the substrate surface with respect to thephoto-absorption part comprising the photo-absorption layer, whereinincident light is refracted at the light incident facet and transits thephoto-absorption layer diagonally with respect to the layer thicknessdirection.

Yet further, a semiconductor photo-detector according to the presentinvention which attains the above object is provided, wherein thesemiconductor layer of large schottky-barrier height isIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1).

Yet further, a semiconductor photo-detector according to the presentinvention which attains the above object is provided, wherein thesemiconductor layer of large schottky-barrier height comprisesIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦x≦1, 0≦y≦1) disposed thereon.

Yet further, a semiconductor photo-detector according to the presentinvention which attains the above object is provided, wherein acompositionally graded or step-graded layer from the same composition asthe photo-absorption layer to the same composition as the semiconductorlayer of large schottky-barrier height is disposed between thephoto-absorption layer and the semiconductor layer of largeschottky-barrier height.

The semiconductor photo-detector according to the present invention ischaracterized in that the light incident facet can be formed very flatand stable as compared with the prior art.

Here, since the light incident facet is formed on the substrate sidegrowth layer except for the photo-absorption layer or on the substratepart from a position apart by a finite length with respect to thephoto-absorption part comprising the photo-absorption layer, etching isprevented to the narrow-gap photo-absorption layer which is relativelyhigher in etching speed during formation of the light incident facet,generation of etching irregularity is almost eliminated, and a flatlight incident facet can be formed with good yield.

Therefore, in the present invention, a narrow-gap photo-absorption layeris not included in the semiconductor layer forming the light incidentfacet, and the narrow-gap photo-absorption layer part does not contactthe light incident facet, there is almost no generation of etchingirregularity such as uneven side etching during deep etching, therebyobtaining a very flat light incident facet.

As a result, diffusion on the facet is prevented for a light beamfocused using a tapered fiber or a lens, beam focusing is maintained,light can be absorbed by a small photo-absorption area, and an ultrafastphoto-detector can be fabricated.

Further, fabrication of the photo-absorption part apart by a finitelength from the light incident facet means that the photo-absorptionpart can be fabricated completely independent of the light incidentfacet, therefore, when light is incident by focusing with a lens or thelike, the photo-absorption part of the device can be decreased in sizeto the same level as the beam size of the focus, thereby enablingultrafast response.

Still further, since, even when the light incident position is moveddown with respect to the top surface, the photo-absorption part can beset at an optimum position without increasing the photo-absorption areain consideration of refraction accordingly, and flexible construction ispossible such as in hybrid integration on a silica-based lightwavecircuit or the like.

Yet further, a semiconductor photo-detector according to the presentinvention which attains the above object is characterized in that: aphoto-absorption part comprising a semiconductor multilayer structureincluding a photo-absorption layer is provided on a substrate, an endsurface is provided with a light incident facet angled inwardly as itseparates from the surface side, and a V- or U-shaped groove is providedin opposition to the light incident facet, whereby light incident froman optical fiber disposed in the groove is refracted at the lightincident facet and transits the photo-absorption layer diagonally withrespect to the layer thickness direction.

Yet further, a production method of semiconductor photo-detectoraccording to the present invention which attains the above object isprovided, wherein the light incident facet and the V- or U-shaped grooveare formed simultaneously by etching.

Yet, further, a semiconductor photo-detector according to the presentinvention which attains the above object is provided, wherein the lightincident facet and its vicinity are buried in an organic substance.

Yet further, production method of semiconductor photo-detection deviceaccording to the present invention which attains the above object isfabricated with the light incident facet and its vicinity are buried inan organic substance, and after making optical coupling with an opticalwaveguide, by removing the organic substance.

Yet further, a production method of semiconductor photo-detection deviceaccording to the present invention which attains the above object ischaracterized in that the light incident facet and its vicinity areburied in an organic substance, and, after making optical coupling withan optical waveguide, space between the semiconductor photo-detector andthe optical waveguide is buried in with an organic substance.

In the semiconductor photo-detection device according to the presentinvention, the device has a groove in opposition to the light incidentfacet to be a fiber guide for conducting incident light, which part actsas a fiber guide, and high precision positioning is possible only bysetting the fiber.

Further, by using a monolithic construction in which the present devicesare arranged in parallel on a single chip, optical coupling is possiblewith multiple fibers collectively with high precision.

Still further, this device has a V- or U-shaped groove in opposition tothe light incident facet, and the light incident facet part and itsvicinity are buried in an organic substance.

Therefore, the V- or U-shaped groove part acts as a guide of opticalwaveguide such as a fiber or the like, and, since the light incidentfacet is protected with the organic substance, high precision opticalcoupling is possible by butting.

Yet further, by using a monolithic construction in which the presentdevices are arranged in parallel on a single chip, optical coupling ispossible with multiple fibers collectively with high precision.

Yet further, a semiconductor photo-detector according to the presentinvention which attains the above object is characterized in that: aphoto-absorption part comprising a semiconductor multilayer structureincluding a photo-absorption layer is provided on a substrate; an endsurface is provided with a light incident facet angled inwardly as itseparates from the surface side, the substrate is protruded by a finitelength from a tip part of the end surface, and light incident from anoptical waveguide, precisely positioned by contacting against theprotruded part of the substrate, is refracted at the light incidentfacet and transits the photo-absorption layer diagonally with respect tothe layer thickness direction.

Since in the device of the invention described above, part of substrateis protruded by a finite length from the tip of the light incidentfacet, this part acts as a stopper when a fiber is brought close from afar end in the optical axis direction, and the fiber tip will nevercontact against the important light incident facet to be damaged.

With the present invention, as compared with the prior art, opticalcoupling in the optical axis direction with a fiber or the like can beachieved precisely without delicate mechanical positioning.

Yet further, a semiconductor photo-detector according to the presentinvention which attains the above object is characterized in that: aphoto-absorption part comprising a semiconductor multilayer structureincluding a photo-absorption layer is provided on a substrate; an endsurface is provided with a light incident facet angled inwardly as itseparates from the surface side, a main reaching area of refractedincident light at the semiconductor layer above the photo-absorptionlayer is terminated with a substance having a smaller refractive indexthan the semiconductor layer, incident light is refracted at the lightincident facet and transits the photo-absorption layer diagonally withrespect to the layer thickness direction, and the transit light is totalreflected by the substance of small refractive index on thesemiconductor layer above the photo-absorption layer.

In the semiconductor photo-detector device of the present invention, amain reaching area of refracted incident light at the semiconductorlayer above the photo-absorption layer is terminated with a substancehaving a smaller refractive index than the semiconductor layer, light iscompletely total reflected on the upper surface, the refracted lighttransits two times the photo-absorption layer, and the effectiveabsorption length is increased to two times.

Therefore, thickness of the photo-absorption layer to obtain a highresponsivity can be considerably reduced.

Due to the remarkable decrease in the photo-absorption layer thickness,ultrafast operation of the device is possible while maintaining a highresponsivity.

A semiconductor photo-detection device according to the presentinvention which attains the above object is characterized by comprisinga refraction type semiconductor photo-detector comprising aphoto-absorption part including a semiconductor multilayer structureincluding a photo-absorption layer disposed on a substrate and an endsurface provided with a light incident facet angle inwardly as itseparates from the surface side, and an optical waveguide disposed inopposition to the device; space between the refraction typesemiconductor photo-detection device and the optical waveguide is buriedin a solid or liquid; whereby light incident to the light incident facetof the photo-detection device from the optical waveguide is refracted atthe light incident facet with respect to the layer thickness direction.

Since in the semiconductor photo-detection device of the presentinvention, space between the refraction type semiconductorphoto-detector and the optical waveguide is buried in a solid or liquidhaving a refractive index of greater than 1, by appropriately selectingthe solid or liquid used to change the refractive index, it is possibleto change the refraction angle on the photo-detector incident facet evenwhen using a refraction type semiconductor photo-detector cut from thesame wafer having the same layer structure and the same mesa angleconstruction thus the responsivity can be adjusted according to theapplication.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a sectional diagram showing an example of prior artsemiconductor photo-detector;

FIG. 1B is a sectional diagram showing another example of prior artsemiconductor photo-detector;

FIG. 2 is a sectional diagram showing a semiconductor photo-detectoraccording to a first embodiment of the present invention;

FIG. 3 is a sectional diagram showing a semiconductor photo-detectoraccording to a second embodiment of the present invention;

FIG. 4 is a sectional diagram showing a semiconductor photo-detectoraccording to a third embodiment of the present invention;

FIG. 5 is a sectional diagram showing a semiconductor photo-detectoraccording to a fourth embodiment of the present invention;

FIG. 6 is a sectional diagram showing a semiconductor photo-detectoraccording to a fifth embodiment of the present invention;

FIG. 7 is a graph showing a calculation result of reverse-mesa angle (θ)dependence of internal quantum efficiency in a 1 μm thick InGaAsphoto-absorption layer to wavelength 1.55 μm light in the fifthembodiment of the present invention;

FIG. 8A is a graph showing a result of incident position dependence ofresponsivity measured when incident beam position is moved in ahorizontal (X) direction in the semiconductor photo-detector of thefifth embodiment of the present invention;

FIG. 8B is a graph showing a result of incident position dependence ofresponsivity measured when incident beam position is moved in a vertical(Y) direction in the semiconductor photo-detector of the fifthembodiment of the present invention;

FIG. 9 is a graph showing frequency response characteristic of a modulefabricated using the semiconductor photo-detector of the fifthembodiment of the present invention at a bias voltage of 5V.

FIG. 10 is a sectional diagram showing a semiconductor photo-detectoraccording to a sixth embodiment of the present invention;

FIG. 11 is a graph showing a calculation result of reverse-mesa angle(θ) dependence of internal quantum efficiency in a 1 μm thick InGaAsphoto-absorption layer to wavelength 1.55 μm light in the sixthembodiment of the present invention;

FIG. 12A is a graph showing a result of incident position dependence ofresponsivity measured when incident beam position is moved in ahorizontal (X) direction in the semiconductor photo-detector of thesixth embodiment of the present invention;

FIG. 12B is a graph showing a result of incident position dependence ofresponsivity measured when incident beam position is moved in a vertical(Y) direction in the semiconductor photo-detector of the sixthembodiment of the present invention;

FIG. 13 is a graph showing frequency response characteristic of a modulefabricated using the semiconductor photo-detector of the sixthembodiment of the present invention at a bias voltage of 5V.

FIG. 14 is a perspective diagram showing a semiconductor photo-detectoraccording to a seventh embodiment of the present invention;

FIG. 15 is a perspective diagram showing a semiconductor photo-detectoraccording to an eighth embodiment of the present invention;

FIG. 16 is a graph showing a calculation result of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees ina InP substrate of the semiconductor photo-detector in the eighthembodiment of the present invention.

FIG. 17 is a perspective diagram showing a semiconductor photo-detectoraccording to a ninth embodiment of the present invention;

FIG. 18 is a sectional perspective diagram of a photo-absorption regionof the semiconductor photo-detector according to the ninth embodiment ofthe present invention;

FIG. 19 is a perspective diagram showing a semiconductor photo-detectoraccording to a tenth embodiment of the present invention;

FIG. 20 is a perspective diagram showing a semiconductor photo-detectoraccording to an eleventh embodiment of the present invention;

FIG. 21 is a perspective diagram showing a semiconductor photo-detectoraccording to a twelfth embodiment of the present invention;

FIG. 22 is a perspective diagram showing a semiconductor photo-detectoraccording to a thirteenth embodiment of the present invention;

FIG. 23 is a sectional perspective diagram of a photo-absorption regionof the semiconductor photo-detector according to the thirteenthembodiment of the present invention;

FIG. 24 is a perspective diagram showing a semiconductor photo-detectoraccording to a fourteenth embodiment of the present invention;

FIG. 25 is a perspective diagram showing a semiconductor photo-detectoraccording to a fifteenth embodiment of the present invention;

FIG. 26 is a perspective diagram showing a semiconductor photo-detectoraccording to a sixteenth embodiment of the present invention;

FIG. 27 is a perspective diagram showing a semiconductor photo-detectoraccording to a seventeenth embodiment of the present invention;

FIG. 28 is a perspective diagram showing a semiconductor photo-detectoraccording to an eighteenth embodiment of the present invention;

FIG. 29 is a graph showing a calculation result of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees ina InP substrate of the semiconductor photo-detector in the eighteenthembodiment of the present invention.

FIG. 30 is a perspective diagram showing a semiconductor photo-detectoraccording to a nineteenth embodiment of the present invention;

FIG. 31 is a perspective diagram showing a semiconductor photo-detectoraccording to a twentieth embodiment of the present invention;

FIG. 32 is a perspective diagram showing a semiconductor photo-detectoraccording to a twenty-first embodiment of the present invention;

FIG. 33 is a perspective diagram showing a semiconductor photo-detectoraccording to a twenty-second embodiment of the present invention;

FIG. 34 is a graph showing a calculation result of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees ina InP substrate of the semiconductor photo-detector in the twenty-secondembodiment of the present invention.

FIG. 35 is a perspective diagram showing a semiconductor photo-detectiondevice according to a twenty-third embodiment of the present invention;

FIG. 36 is a graph showing a calculation result of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees ina InP substrate of the semiconductor photo-detector in the twenty-thirdembodiment of the present invention.

FIG. 37 is an enlarged diagram of the light incident facet part of thesemiconductor photo-detection device according to the twenty-thirdembodiment of the present invention;

FIG. 38 is a perspective diagram showing a semiconductor photo-detectiondevice according to a twenty-fourth embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiment 1

A first embodiment of the present invention is shown in FIG. 2.

In FIG. 2, numeral 11 indicates a light incident facet, 12 is a 1 μmthick p-InP layer (first semiconductor layer), 13 is a 1.0 μm thickInGaAs photo-absorption layer, 14 is a 1 μm thick n-InP layer (secondsemiconductor layer), 15 is a semi-insulating InP substrate, 16 is ap-electrode, and 17 is an n-electrode. The device has a photo-absorptionlayer area of 10 μm×20 μm.

The p-InP layer 12 and the photo-absorption layer 13 are formed on apart which is separated inside by a finite length (5 μm in thisembodiment) in a direction parallel to the substrate surface from then-InP layer 14 and an end surface 500 of the substrate. The lightincident facet 11 is formed in a shape angled inwardly as it separatesfrom the n-InP layer 14 or the surface of the substrate 15.

A production method of the photo-detector formed in the shape mentionedabove will be described as follows.

In the present embodiment, the light incident facet 11 was formed byutilizing wet etching with bromine-methanol of a (001) surface wafer toexpose a (111)A plane.

Since, in this case, reverse-mesa etching is performed on the n-InPlayer 14 and the semi-insulating InP substrate which are comprised onlyInP, the uniform, flat angled light incident facet 11 can be formed witha good yield.

Further, an etching mask is formed at a position 8 μm apart from thephoto-absorption part including the photo-absorption layer 13, and deepreverse-mesa etching of about 30 μm is performed, in this case, sideetching of about 3 μm occurs, however, in the photo-absorption part doesnot contact the side etching part, therefore, an abnormal side etching,etching irregularity or the like caused by relatively fast etching speedof the photo-absorption layer 13 will not generate. In addition, uniformdevices with equal mesa angle can be fabricated.

Naturally, the reverse-mesa may be formed using another wet etchingliquid or dry etching, or utilizing other crystal plane, or utilizingadhesion of the etching mask to control the angle.

By any method, since the etching object is only InP layer of uniformcomposition, etching irregularity is hard to occur, and a flat lightincident facet 11 can be formed with a good yield.

Because a very flat light incident facet 11 can be formed as describedabove, by forming an anti-reflection coating film on the light incidentfacet 11, in light of 1.3 μm in wavelength of focused beam diameter by atapered fiber, the refracted light is introduced into thephoto-absorption part with good focus without diffusion of the beam onthe light incident facet 11, and a high responsivity of more than 0.8A/W is obtained at an applied reverse bias of 1.5V.

Further, since photo-absorption can be achieved with such a small-sizedphoto-absorption area, high speed operation of a 3 dB bandwidth of 40GHz was possible.

As described above, fabrication of the photo-absorption part apart fromthe light incident facet 11 means that the photo-absorption part can befabricated completely independent of the light incident facet 11,therefore, when light is incident by focusing with a lens or the like,the photo-absorption part of the device can be small-sized to the samelevel as the beam size of focus, thereby enabling ultrafast response.

Still further, since, even when the light incident position is moveddown to the substrate side relative to the top surface, thephoto-absorption part can be set at an optimum position withoutincreasing the photo-absorption area in consideration of refraction, inhybrid integration on a PLC (Planar Lightwave Circuit) which is asilica-based lightwave circuit, flexible construction is possiblewithout degradation of high-speed performance due to a change of opticalaxis position.

The present embodiment is an example using an n-InP layer at thesubstrate side, however, a p-InP layer can be used by reversing theabove p and n, and an n-InP or p-InP substrate can also be used for thefabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer 13, however, it is needless to say that a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 2

A second embodiment of the present invention is shown in FIG. 3.

In FIG. 3, numeral 31 indicates a light incident facet, 32 is a 1 μmthick InP layer (first semiconductor layer), 32-2 is a p-InP layer(first semiconductor layer) formed by Zn diffusion, 33 is a 1.0 μm thickInGaAs photo-absorption layer, 34 is a 1 μm thick n-InP layer (secondsemiconductor layer), 35 is a semi-insulating InP substrate, 36 is a pelectrode, and 37 is an n electrode. The device has a photo-absorptionlayer area of 10 μm×20 μm.

Also in the construction shown in FIG. 3, configuration relation betweenthe photo-absorption layer 33 and the light incident facet 31 is thesame as in the above-described embodiment 1 (FIG. 2).

In the present embodiment, the light incident facet 31 was formed byutilizing wet etching of a (001) surface wafer using bromine-methanol toexpose a (111)A plane.

Since, in this case, reverse-mesa etching is performed by wet etching tothe n-InP layer 34 and the InP substrate 35 composed only of InP, auniform angled light incident facet 31 of good flatness can be formedwith a good yield.

Further, an etching mask is formed at a position 8 μm apart from thephoto-absorption part including the photo-absorption layer 33, and deepreverse-mesa etching of about 30 μm is performed, in this case, sideetching of about 3 μm occurs, however, in the photo-absorption part doesnot contact the side etching part, therefore, an abnormal side etching,etching irregularity or the like caused by relatively fast etching speedof the photo-absorption layer 33 will not generate. In addition, uniformdevices with equal mesa angle can be fabricated.

Naturally, the reverse-mesa may be formed using another wet etchingliquid or dry etching, or utilizing other crystal plane, or utilizingadhesion of the etching mask to control the angle.

By any method, since the etching object is only InP layer of uniformcomposition, etching irregularity is hard to occur, and a flat lightincident facet 31 can be formed with a good yield.

Because a very flat light incident facet 31 can be formed as describedabove, by forming an anti-reflection coating film on the light incidentfacet 31, in light of 1.3 μm in wavelength of focused beam diameter by atapered fiber, the refracted light is introduced into thephoto-absorption part with good focus without diffusion of the beam onthe light incident facet 31, and a high responsivity of more than 0.8A/W was obtained at an applied reverse bias of 1.5V.

A dark current of a sufficiently small value of about 10 pA was obtainedeven after formation of the anti-reflection coating film.

Further, since photo-absorption can be achieved with such a small-sizedphoto-absorption area, high speed operation of a 3 dB bandwidth of 40GHz was possible.

As described above, fabrication of the photo-absorption part apart fromthe light incident facet 31 means that the photo-absorption part can befabricated completely independent of the light incident facet 31,therefore, when light is incident by focusing with a lens or the like,the photo-absorption part of the device can be small-sized to the samelevel as the beam size of focus, thereby enabling ultrafast response.

Still further, since, even when the light incident position is moveddown to the substrate side relative to the top surface, thephoto-absorption part can be set at an optimum position withoutincreasing the photo-absorption area in consideration of refraction, inhybrid integration on a PLC which is a silica-based lightwave circuit,flexible construction is possible without degradation of high-speedperformance due to a change of optical axis position.

In the present embodiment, the conduction type of semiconductor of themain part at the surface side is determined by Zn diffusion, however,alternatively, it may be determined by an ion implantation method andsubsequent anneal.

The present embodiment is an example using an n-InP layer at thesubstrate side, however, a p-InP layer can be used by reversing theabove p and n, and an n-InP or p-InP substrate can also be used for thefabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer 33, however, it is needless to say that a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 3

A third embodiment of the present invention is shown in FIG. 4.

In FIG. 4, numeral 41 indicates a light incident facet, 42 is a 0.2 μmthick undoped or n⁻-InAlAs layer, 43 is a 0.1 μm thick undoped orn⁻-In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) in which the composition issmoothly changed from InAlAs to InGaAs, 44 is a 1 μm thick undoped orn⁻-InGaAs photo-absorption layer, 45 is a 1 μm thick n-InP layer, 46 isa semi-insulating InP substrate, 47 is a Pt/Ti/Au schottky electrode,and 48 is an ohmic n electrode. The device has a photo-absorption layerarea of 10 μm×20 μm.

Also in the construction shown in FIG. 4, configuration relation betweenthe photo-absorption layer 44 and the light incident facet 41 is thesame as in embodiment 1 described above (FIG. 2).

Because light transits obliquely with respect to the photo-absorptionlayer 44 due to refraction of light on the light incident facet 41, theeffective absorption length is increased.

Further, because the schottky electrode 47 acts as a reflection mirrorto refracted incident light so that the absorption length equivalentlybecomes further two times, by forming an anti-reflection coating film onthe light incident facet 41 with a photo-absorption layer thickness of 1μm, a large responsivity of more than 0.9 A/W at a wavelength of 1.3 μmwas obtained at an applied reverse bias of 1.5V.

In the present embodiment, the light incident facet 41 was formed byutilizing wet etching of a (001) surface wafer using bromine-methanol toexpose a (111)A plane.

Since, in this case, reverse-mesa etching is performed on the n-InPlayer 45 and the InP substrate 46 which are composed only of InP, auniform angled light incident facet 41 of good flatness can be formedwith a good yield.

Further, an etching mask is formed at a position 8 μm apart from thephoto-absorption part including the photo-absorption layer 44, and deepreverse-mesa etching of about 30 μm is performed, in this case, sideetching of about 3 μm occurs, however, the photo-absorption part doesnot contact the side etching part, therefore, abnormal side etching,etching irregularity or the like caused by relatively fast etching speedof the photo-absorption layer 44 will not generate. In addition, uniformdevices with equal mesa angle can be fabricated.

Naturally, the reverse-mesa may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle.

By any of the methods, since the etching object is only InP layer ofuniform composition, etching irregularity is hard to generate, and aflat light incident facet 41 can be formed with a good yield.

Because a very flat light incident facet 41 can be formed as describedabove, by forming an anti-reflection coating film on the light incidentfacet 41, for light of 1.3 μm in wavelength of focused beam diameter bya tapered fiber, the refracted light is introduced into thephoto-absorption part with good focus without diffusion of the beam onthe light incident facet 41.

A dark current of a sufficiently small value of about 1 nA was obtainedeven after formation of the anti-reflection coating film.

Further, since photo-absorption can be achieved with such a small-sizedphoto-absorption area, high speed operation of a 3 dB bandwidth of 30GHz was possible.

As described above, fabrication of the photo-absorption part apart fromthe light incident facet 41 means that the photo-absorption part can befabricated completely independent of the light incident facet 41,therefore, when light is incident by focusing with a lens or the like,the photo-absorption part of the device can be small-sized to the samelevel as the beam size of focus, thereby enabling ultrafast response.

Still further, since, even when the light incident position is moveddown to the substrate side relative to the top surface, thephoto-absorption part can be set at an optimum position withoutincreasing the photo-absorption area in consideration of refraction, inhybrid integration on a PLC which is a silica-based lightwave circuit,flexible construction is possible without degradation of high-speedperformance due to a change of optical axis position.

In the present embodiment, for smooth connection of the conduction bandas well as the valence band, a compositionally graded layer 43 is usedin which composition is smoothly changed from InAlAs to InGaAs, however,as the layer 43, a quasi-graded layer may be used which is composed of astep-graded layer including multilayered semiconductor thin films ofmore than one layer.

Further, also between the layers 44 and 45, smooth connection of boththe conduction band and valence band may be achieved usingIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) step-graded layer in whichcomposition is changed from InGaAs to InP or a quasi-graded layer.

Yet further, as a semiconductor photo-detection device, on asemiconductor layer having a first conduction type, a multilayerstructure having a large schottky-barrier height opposing a schottkyelectrode 47 which has a schottky-barrier higher than the schottkybarrier between the photo-absorption layer 44 and the schottky electrode47 may be constructed on the substrate, between the photo-absorptionlayer 44 comprising an intrinsic or first conduction type semiconductorlayer, a superlattice semiconductor layer, or a multiple quantum wellsemiconductor layer and the schottky electrode 47.

Yet further, as the semiconductor layer of large schottky barrierheight, In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) can be used.

The present embodiment is an example using an n-InP layer at thesubstrate side, however, a p-InP layer can be used by reversing theabove p and n, and an n-InP or p-InP substrate can also be used for thefabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer 44, however, it is needless to say that a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 4

A fourth embodiment of the present invention is shown in FIG. 5.

In FIG. 5, numeral 51 indicates a light incident facet, 59 is a 5 nmthick undoped or n⁻-InP layer, 52 is a 0.2 μm thick undoped or n⁻-InAlAslayer, 53 is a 0.1 μm thick undoped or n⁻-In_(1-x-y)Ga_(x)Al_(y)As(0≦x≦1, 0≦y≦1) layer in which composition is smoothly changed fromInAlAs to InGaAs, 54 is a 1 μm thick undoped or n-InGaAsphoto-absorption layer, 55 is a 1 μm thick n-InP layer, 56 is asemi-insulating InP substrate, 57 is a Pt/Ti/Au schottky electrode, and58 is an ohmic n electrode. The device has a photo-absorption layer areaof 10 μm×20 μm.

Also in the construction shown in FIG. 5, configuration relation betweenthe photo-absorption layer 54 and the light incident facet 51 is thesame as in embodiment 1 described above (FIG. 2).

Since, in the present embodiment, a very thin InP layer 59 is used onthe top surface, it has a high surface oxidation resistance as comparedwith InAlAs.

Because light transits obliquely with respect to the photo-absorptionlayer 54 due to reflection of light on the light incident facet 51, theeffective absorption length is increased.

Further, because the schottky electrode 57 acts as a reflection mirrorto refracted incident light so that the absorption length is increasedequivalently by two times, by forming an anti-reflection coating film onthe light incident facet 51 with a photo-absorption layer thickness of 1μm, a large responsivity of more than 0.9 A/W at a wavelength of 1.3 μmwas obtained at an applied reverse bias of 1.5V.

In the present embodiment, the light incident facet 51 was formed byutilizing wet etching of a (001) surface wafer using bromine-methanol toexpose a (111)A plane.

Since, in this case, reverse-mesa etching is performed on the n-InPlayer 55 and the InP substrate 56 which are composed only of InP, auniform angled light incident facet 51 of good flatness can be formedwith a good yield.

Further, an etching mask is formed at a position 8 μm apart from thephoto-absorption part including the photo-absorption layer 54, and deepreverse-mesa etching of about 30 μm is performed, in this case, sideetching of about 3 μm occurs, however, the photo-absorption part doesnot contact the side etching part, therefore, abnormal side etching,etching irregularity or the like caused by relatively fast etching speedof the photo-absorption layer 54 will not generate. In addition, uniformdevices with equal mesa angle can be fabricated.

Naturally, the reverse-mesa may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle.

By any of the methods, since the etching object is only InP layer ofuniform composition, etching irregularity is hard to generate, and aflat light incident facet 51 can be formed with a good yield.

Because a very flat light incident facet 51 can be formed as describedabove, by forming an anti-reflection coating film on the light incidentfacet 51, for light of 1.3 μm in wavelength of focused beam diameter bya tapered fiber, the refracted light is introduced into thephoto-absorption part with good focus without diffusion of the beam onthe light incident facet 51.

A dark current of a sufficiently small value of about 1 nA was obtainedeven after formation of the anti-reflection coating film.

Further, since photo-absorption can be achieved with such a small-sizedphoto-absorption area, high speed operation of a 3 dB bandwidth of 30GHz was possible.

As described above, fabrication of the photo-absorption part apart fromthe light incident facet 51 means that the photo-absorption part can befabricated completely independent of the light incident facet 51,therefore, when light is incident by focusing with a lens or the like,the photo-absorption part of the device can be small-sized to the samelevel as the beam size of focus, thereby enabling ultrafast response.

Still further, since, even when the light incident position is moveddown to the substrate side relative to the top surface, thephoto-absorption part can be set at an optimum position withoutincreasing the photo-absorption area in consideration of refraction, inhybrid integration on a PLC which is a silica-based lightwave circuit,flexible construction is possible without degradation of high-speedperformance due to a change of optical axis position.

In the present embodiment, for smooth connection of the conduction bandas well as the valence band, a compositionally graded layer 53 is usedin which composition is smoothly changed from InAlAs to InGaAs, however,as the layer 53, a quasi-graded layer may be used which is composed of astep-graded layer including multilayered semiconductor thin films ofmore than one layer.

Further, also between the layers 54 and 55, smooth connection of boththe conduction band and valence band may be achieved usingIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) step-graded layer in whichcomposition is changed from InGaAs to InP or a quasi-graded layer.

Yet further, as a semiconductor photo-detection device, on asemiconductor layer having a first conduction type, a multilayerstructure having a large schottky-barrier height opposing a schottkyelectrode 57 which has a schottky-barrier higher than the schottkybarrier between the photo-absorption layer 54 and the schottky electrode57 may be constructed on the substrate, between the photo-absorptionlayer 54 comprising an intrinsic or first conduction type semiconductorlayer, a superlattice semiconductor layer, or a multiple quantum wellsemiconductor layer and the schottky electrode 57.

Yet further, as the semiconductor layer of large schottky barrierheight, In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) can be used, and it maybe constructed with a semiconductor comprising In_(1-x-y)Ga_(x)Al_(y)As(0≦x≦1, 0≦y≦1) and thin In_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1)disposed thereon.

The present embodiment is an example using an n-InP layer at thesubstrate side, however, a p-InP layer can be used by reversing theabove p and n, and an n-InP or p-InP substrate can also be used for thefabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer 54, however, it is needless to say that a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 5

A fifth embodiment of the present invention is shown in FIG. 6.

In FIG. 6, numeral 61 indicates a light incident facet, 62 (firstsemiconductor layer) is a p-type semiconductor layer composed of fromthe upper layer, a 0.1 μm thick p-InGaAs contact layer, a 1.3 μm thickp-InP layer, and a 0.2 μm thick p-InGaAsP layer (1.15 μm wavelengthcomposition), 63 is a 1.0 μm thick InGaAs photo-absorption layer, 64(second semiconductor layer) is an n-type semiconductor layer composedof from the upper layer, a 2 μm thick n-InGaAsP layer (1.15 μmwavelength composition) and 0.2 μm thick InP layer, 65 is asemi-insulating InP substrate, 66 is a p electrode, and 67 is an nelectrode. The device has a photo-absorption layer area of 14 μm×20 μm.

Also in the construction shown in FIG. 6, configuration relation betweenthe photo-absorption layer 63 and the light incident facet 61 is thesame as in embodiment 1 described above (FIG. 2).

FIG. 7 is a calculation result of reverse-mesa angle (θ) dependence ofinternal quantum efficiency in the 1 μm thick InGaAs photo-absorptionlayer to wavelength 1.55 μm light. As the reverse-mesa angle (θ)increases, the effective absorption length increases and the internalquantum efficiency increases.

Further, in the present embodiment, the light incident facet 61 wasformed by utilizing wet etching of a (001) surface wafer usingbromine-methanol to form a (111)A plane with an angle of 54°44′ relativeto the surface.

Since, in this case, reverse-mesa etching is performed on the InPsubstrate 65 which is composed only of InP, a uniform angled lightincident facet of good flatness can be formed with a good yield.

Further, an etching mask is formed at a position 8 μm apart from thephoto-absorption part including the photo-absorption layer 63, and deepreverse-mesa etching of about 30 μm is performed, in this case, sideetching of about 3 μm occurs, however, the photo-absorption part doesnot contact the side etching part, therefore, abnormal side etching,etching irregularity or the like caused by relatively fast etching speedof the photo-absorption layer will not generate. In addition, uniformdevices with equal mesa angle can be fabricated.

Naturally, the reverse-mesa may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle.

By any of the methods, since the etching object is only InP layer ofuniform composition, etching irregularity is hard to generate, and aflat light incident facet 61 can be formed with a good yield.

Because a very flat light incident facet 61 can be formed as describedabove, by forming an anti-reflection coating film on the light incidentfacet, for light of 1.55 μm in wavelength of output light from the fiberwith a spot-size diameter (2 Ws) of 3.4 μm focused by a lens, therefracted light is introduced into the photo-absorption part with goodfocus without diffusion of the beam on the light incident facet, and alarge responsivity value of 1.0 A/W was obtained at an applied reversebias of 5V.

Still further, the incident position of beam was moved in the horizontal(X) and vertical (Y) directions and measured for incident positiondependence of responsivity. The measurement results are shown in FIGS.8A and 8B. Optical axis misalignment tolerance of decreasing theresponsivity by 1 dB is as large as 13.4 μm in the horizontal directionand 3.3 μm in the vertical direction. As a result, a photo-absorptionmodule could be fabricated by a simple single-lens constructionproviding a cost reduction. Most of fabricated modules showed highresponsivities of 0.8 to 1.0 A/W.

FIG. 9 shows frequency response characteristic of the fabricated moduleat a bias voltage of 5V. Since photo-absorption can be achieved with asmall-sized photo-absorption area, high speed operation of a 3 dBbandwidth of 38 GHz was possible.

The present embodiment is an example using an n-type semiconductor layerat the substrate side, however, a p-type semiconductor layer can be usedby reversing the above p and n, and an n-InP or p-InP substrate can alsobe used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 6

A sixth embodiment of the present invention is shown in FIG. 10.

In FIG. 10, numeral 71 indicates a light incident facet, 72 is a p-typesemiconductor layer (first semiconductor layer) composed of, from theupper layer, a 0.1 μm thick p-InGaAs contact layer, a 1.3 μm thick p-InPlayer, and a 0.2 μm thick p-InGaAsP layer (1.15 μm wavelengthcomposition), 73 is a 1.0 μm thick InGaAs photo-absorption layer, 74 isa 2 μm thick n-InP layer (second semiconductor layer), 75 is asemi-insulating InP substrate, 76 is a p electrode, and 77 is an nelectrode. The device has a photo-absorption layer area of 14 μm×20 μm.

Also in the construction shown in FIG. 10, configuration relationbetween the photo-absorption layer 73 and the light incident facet 71 isthe same as in embodiment 1 described above (FIG. 2).

FIG. 11 is a calculation result of reverse-mesa angle (θ) dependence ofinternal quantum efficiency in the 1 μm thick InGaAs photo-absorptionlayer to wavelength 1.55 μm light. As the reverse-mesa angle (θ)increases, the effective absorption length increases and the internalquantum efficiency increases.

Further, in the present embodiment, the light incident facet 71 wasformed by utilizing wet etching of a (001) surface wafer usingbromine-methanol to form a (111)A plane with an angle of 54°44′ relativeto the surface.

Since, in this case, reverse-mesa etching is performed on a partincluding the InP substrate 75 which is composed only of InP, a uniformangled light incident facet 71 of good flatness can be formed with agood yield.

Further, an etching mask is formed at a position 8 μm apart from thephoto-absorption part including the photo-absorption layer, and deepreverse-mesa etching of about 30 μm is performed, in this case, sideetching of about 3 μm occurs, however, the photo-absorption part doesnot contact the side etching part, therefore, abnormal side etching,etching irregularity or the like caused by relatively fast etching speedof the photo-absorption layer will not generate. In addition, uniformdevices with equal mesa angle can be fabricated.

Naturally, the reverse-mesa may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle. By any ofthe methods, since the etching object is only InP layer of uniformcomposition, etching irregularity is hard to generate, and a flat lightincident facet 71 can be formed with a good yield.

Because a very flat light incident facet 71 can be formed as describedabove, by forming an anti-reflection coating film on the light incidentfacet, for light of 1.55 μm in wavelength of output light from the fiberwith a spot-size diameter (2 Ws) of 3.4 μm focused by a lens, therefracted light is introduced into the photo-absorption part with goodfocus without diffusion of the beam on the light incident facet, and alarge responsivity value of 1.0 A/W was obtained at an applied reversebias of 5V.

Still further, the incident position of beam was moved in the horizontal(X) and vertical (Y) directions and measured for incident positiondependence of responsivity. The measurement results are shown in FIGS.12A and 12B. Optical axis misalignment tolerance of decreasing theresponsivity by 1 dB is as large as 13.4 μm in the horizontal directionand 3.3 μm in the vertical direction. As a result, a photo-absorptionmodule could be fabricated by a simple single-lens constructionproviding a cost reduction. Most of fabricated modules showed highresponsivities of 0.8 to 1.0 A/W.

FIG. 13 shows frequency response characteristic of the fabricated moduleat a bias voltage of 5V. Since photo-absorption can be achieved with asmall-sized photo-absorption area, high speed operation of a 3 dBbandwidth of 38 GHz was possible.

The present embodiment is an example using an n-type semiconductor layerat the substrate side, however, a p-type semiconductor layer can besimilarly used by reversing the above p and n, and an n-InP or p-InPsubstrate can also be used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 7

A seventh embodiment of the present invention is shown in FIG. 14.

In FIG. 14, numeral 81 indicates a light incident facet, 82 is a 1 μmthick p-InP layer, 83 is a 1 μm thick InGaAs photo-absorption layer, 84is a 1 μm thick n-InP layer, 85 is a semi-insulating InP substrate, 86is a p electrode, 87 is an n electrode, and 88 is a V-shaped groove. Thedevice has a photo-absorption layer area of 30 μm×50 μm.

The light incident facet 81 and the V-shaped groove 88 weresimultaneously formed using a silicon nitride film mask having aT-shaped window by wet etching with bromine-methanol.

In this case, the light incident facet 81 and the V-shaped groove 88, inwet etching of a (001) surface wafer using bromine-methanol, were formedutilizing the property that the (111)A plane is formed in a reverse-mesashape, and in a direction perpendicular thereto, the (−1-11) and (111)planes are formed in a forward mesa shape.

In this case, the V-shaped groove 88 is fabricated with good precisionin the plane direction and, therefore, the depth can also be controlledwith good controllability by the window width of the mask.

Naturally, the light incident facet (reverse-mesa part) 81 and theV-shaped groove 88 may be formed using another wet etching liquid or adry etching method, or utilizing other crystal plane, or utilizingadhesion of the etching mask to control the angle.

Since the V-shaped groove is automatically formed by the fact that theright and left forward mesa surfaces go down with the passage of etchingtime until base lines of both sides are in line with each other, thedepth can also be controlled with good controllability by the maskwindow width, however, before both base lines fall in line, the grooveis in a so-called U shape. Therefore, by controlling the etching depthwith time, a U-shaped groove can also be formed as necessary. Further,U-shaped grooves of different shapes can be formed by changing toanother etching liquid in the course of etching.

When an anti-reflection coating film was formed on the light incidentfacet 81, a single mode fiber was guided by the V-shaped groove, andlight of 1.3 μm was introduced, a large responsivity value of more than0.8 A/W was obtained at an applied reverse bias of 1.5V.

Here, since the light incident position can be almost determined by theprecision of the mask for forming the V-shaped groove 88, withoutpositioning by mechanical movement of fiber, high precision positioningwas possible.

Since high precision positioning is possible even when the beam size isdecreased using a tapered fiber in place of the single mode fiber, thephoto-absorption part of the device can be small-sized to the same levelas the beam size of the focus.

Therefore, a device capable of achieving ultrafast response by sizereduction can be realized in a high precision condition with the fiber.

With a device of a photo-absorption area of (10 μm×20 μm), high speedoperation of a 3 dB bandwidth of 40 GHz was possible while maintaining ahigh responsivity.

As described above, since the photo-absorption part and the fiber can becoupled in high precision without a lens system, a module could also beeasily fabricated.

In the present embodiment, the p-InP layer at the surface side is formedby a crystal growth method, however, alternatively, an undoped InP maybe formed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal.

Yet further, as a semiconductor photo-detector, on a semiconductor layerhaving a first conduction type, a multilayer structure having a largeschottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer 83 and the schottky electrode may be constructedon the substrate 85, between the photo-absorption layer 83 comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode.

Yet further, a semiconductor photo-detector may be constructed with thesemiconductor layer of large schottky barrier height comprisingIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) or In_(1-x-y)Ga_(x)Al_(y)As(0≦x≦1, 0≦y≦1) and thin In_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1)disposed thereon.

Yet further, the present embodiment is an example using semi-insulatingInP as the substrate 85 and an n-InP layer at the substrate side,however, a p-InP layer can be used by reversing the above p and n, andan n-InP or p-InP substrate can also be used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer 83, however, it is needless to say that a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 8

FIG. 15 is a diagram for describing an eighth embodiment of the presentinvention. Numeral 101 indicates a light incident facet, 102 is a 1 μmp-InP layer, 103 is a 0.7 μm thick InGaAs photo-absorption layer, 104 isa 1 μm thick n-InP layer, 105 is a semi-insulating InP substrate, 106 isa p electrode, 107 is an n electrode, 108 is a V-shaped groove, 109 isan optical fiber, and 110 is polyimide. The device has aphoto-absorption layer area of 30 μm×70 μm. The light incident facet andthe V-shaped groove 108 were simultaneously formed by wet etching withbromine-methanol using a silicon nitride film mask having a T-shapedwindow. At this moment, the light incident facet 101 and the V-shapedgroove 108 were formed utilizing the property that the (111)A plane isformed in reverse-mesa shape as shown in the figure and the (−1-11) and(111) planes perpendicular thereto are formed in forward mesa shape inwet etching of a (001) surface wafer with bromine-methanol. In thiscase, the V shape is fabricated with good precision in the planedirection and, therefore, the depth can also be controlled with goodcontrollability by the window width of the mask. Naturally, thereverse-mesa part and the V-shaped groove 108 may be formed usinganother wet etching liquid or a dry etching method, or utilizing othercrystal plane, or utilizing adhesion of the etching mask to control theangle. Further, the V-shaped groove 108 can be formed in a U shape orthe like by controlling the etching condition and the mask shape.

The single mode fiber 109 is guided by the V-shaped groove 108, disposedopposite to the light incident facet as shown in the figure, and thespace is buried in polyimide having a refractive index of 1.7. Thephoto-detector and the fiber end surface are provided withanti-reflection coating films. When light of wavelength 1.55 μm wasintroduced by the single mode fiber 109, a large responsivity value of1.0 A/W at an applied reverse bias of 1.5V is obtained. Here, since thelight incident position can be almost determined by the precision of themask for forming the V-shaped groove guide, without positioning bymechanical movement of fiber, high precision positioning was possible.Since, as shown above, the photo-absorption part and the fiber 109 canbe coupled with high precision without a lens system, a module could beeasily fabricated. Layer structure of the photo-detector of the presentembodiment is designed for enabling high speed operation. That is, thephoto-absorption layer is as thin as 0.7 μm to decrease the carriertransit time. Further, the refraction angle is the largest when thespace between the device and the fiber is air, therefore, the devicelength required for receiving refracted light can be reduced, therebyreducing the device capacity which is determined by the device size.Since, even when a tapered fiber is used in place of the single modefiber 109 to decrease the beam size, the photo-absorption part of thedevice can be small-sized to the same level as the focus beam size.Therefore, a device capable of making ultrafast response by sizereduction can be realized with a high-coupling state with the fiber.With a device of a photo-absorption area of 10 μm×20 μm, high speedoperation of more than a 3 dB bandwidth of 40 GHz was possible whilemaintaining high responsivity. However, with the device using airbetween the device and fiber, the responsivity was 0.8 A/W. This isbecause absorption in the photo-absorption layer is insufficient. Sincehigh speed and responsivity are basically in a trade-off relation, adecrease in responsivity associated with high speed is inevitable tosome extent, however, according to the present invention, it is possibleto change the responsivity over a wide range even when the absorptionlayer is decreased in thickness for high speed operation. However, theoperation speed is decreased to some extent by increasing the devicecapacity associated with an increase in photo-detector length.

FIG. 16 shows a calculation result in InP of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of the substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees.The extension factor can be substantially increased by using a materialhaving a large n₁. In the present embodiment, polyimide having arefractive index of 1.7 is used, the extension factor in this caseprovides a 41% increase relative to air, as a result, the above increasein responsivity is obtained. It is needless to say that the substancebetween the device and fiber may be any one which has a goodtransmissivity to incident light and a relative index of more than 1,selected from organic substances such as various polyimides includingfluorinated polyimides, various epoxies including epoxy resins,fluorinated epoxies, fluorinated epoxy acrylate resins, acrylics,metamorphic silicone resins, and inorganic substances such aschalcogenide glass having a refractive index of more than 2, or a liquidsubstance such as oil. By appropriately selecting such varioussubstances, it is possible to change the refraction angle at the lightincident facet of the photo-detector, even when using a refraction typesemiconductor photo-detector cut from the same wafer having the samelayer structure and the same mesa angle construction, and adjustment ofresponsivity according to the application is possible.

Further, as to the optical fiber, in the present embodiment, a singlemode optical fiber or a tapered fiber is used, however, it is needlessto say that one which is based on various organic substances such as aplastic fiber may be used.

In the present embodiment, the p-InP layer at the surface side is formedby a crystal growth, however, alternatively, an undoped InP may beformed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal. Further, as the semiconductorphoto-detector, on a semiconductor layer having a first conduction type,a multilayer structure having a large schottky-barrier height opposing aschottky electrode which has a schottky-barrier higher than the schottkybarrier between the photo-absorption layer and the schottky electrodemay be constructed on the substrate, between the photo-absorption layercomprising an intrinsic or first conduction type semiconductor layer, asuperlattice semiconductor layer, or a multiple quantum wellsemiconductor layer and the schottky electrode, and a semiconductorphoto-detector may be constructed with the semiconductor layer of largeschottky barrier height comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1,0≦y≦1) or In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon. Yetfurther, the present embodiment is an example using semi-insulating InPas the substrate and an n-InP layer at the substrate side, however, ap-InP layer can be used by reversing the above p and n, and an n-InP orp-InP substrate can also be used for the fabrication. Yet further, auniform composition bulk is used here as the photo-absorption layer,however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used. Yet further, it is needless to saythat a material system other than InGaAsP/InP such as InGaAlAs/InGaAsPor AlGaAs/GaAs system or a material system including a strain may beused.

Embodiment 9

FIG. 17 is a diagram for describing a ninth embodiment of the presentinvention. FIG. 18 is a sectional perspective diagram of thephoto-absorption region. Numeral 111 indicates a light incident facet,112 is a 0.2 μm thick p⁺-InGaAsP (1.2 μm composition) layer, 113 is a 1μm thick p-InP layer, 114 is a 0.5 μm thick InGaAs photo-absorptionlayer, 115 is a 1 μm thick n-InP layer, 116 is a semi-insulating InPsubstrate, 117 is a p electrode, 118 is an n electrode, and 119 is aV-shaped groove. A lead electrode and a pad electrode are omitted inthis figure for simplicity since these parts are complicated instructure and intricate to be described. The device has aphoto-absorption layer area of 30 μm×50 μm. The light incident facet 111and the V-shaped groove 119 were simultaneously formed by wet etchingwith bromine-methanol using a silicon nitride film mask having aT-shaped window. At this moment, the light incident facet 111 and theV-shaped groove 119 were formed utilizing the property that the (111)Aplane is formed in reverse-mesa shape as shown in the figure and the(−1-11) and (111) planes perpendicular thereto are formed in forwardmesa shape in wet etching of a (001) surface wafer withbromine-methanol. In this case, the V shape is fabricated with goodprecision in the plane direction and, therefore, the depth can also becontrolled with good controllability by the window width of the mask.

Naturally, the reverse-mesa part and the V-shaped groove 119 may beformed using another wet etching liquid or a dry etching method, orutilizing other crystal plane, or utilizing adhesion of the etching maskto control the angle. Further, the V-shaped groove 119 can be formed ina U shape or the like by controlling the etching condition and the maskshape.

An anti-reflection coating film is formed on the light incident facet.Right above the main reaching area of refracted light is terminated withair, and the p electrode 117 is formed in outer peripheral part of thearea. Incident light of wavelength 1.3 μm from the lateral side isrefracted at the light incident facet and transmits at an angle ofφ=24.7 degrees as shown in the figure relative to the upper surface. Inthis case, if the refractive index of the semiconductor is assumed as3.209, total reflection occurs when φ is smaller than 71.8 degrees, andthe present embodiment satisfies this condition. When the single modefiber was guided by the V-shaped groove 119 and light of wavelength 1.3μm was incident, a large responsivity value of more than 0.9 A/W wasobtained at an applied reverse bias of 1.0V. In the case of prior artstructure in which the p electrode is present on almost the entiresurface of the upper surface, a value of only about 0.7 A/W wasobtained. Here, since the light incident position can be almostdetermined by the precision of the mask for forming the V-shaped grooveguide, without positioning by mechanical movement of the fiber, highprecision positioning was possible. Since, even when a tapered fiber isused in place of the single mode fiber to decrease the beam size, thephoto-absorption part of the device can be small-sized to the same levelas the focus beam size. Therefore, a device capable of making ultrafastresponse by size reduction can be realized with a high-coupling statewith the fiber. With a device of a photo-absorption area of 10 μm×20 μm,high speed operation of more than a 3 dB bandwidth of 40 GHz waspossible while maintaining high responsivity. Since, as shown above, thephoto-absorption part and the fiber can be coupled with high precisionwithout a lens system, a module could be easily fabricated.

In the present embodiment, the p electrode is formed in a ring shape,however, it is needless to say that although the overall reflectivity isslightly decreased, it may be formed in any structure such as areticulated electrode shape, or the upper InP/InGaAsP layer remained asa high-concentration doped layer to decrease the sheet resistancesufficiently, or the p electrode be formed in only one side part. In thepresent embodiment, right above the reaching area of incident light isterminated with air, it may be any of inorganic substances such as SiO₂or SiNx or organic substances such as polyimide or epoxy, which has anappropriately smaller refractive-index than the semiconductor andsatisfies the total reflection condition (φ<cos⁻¹(n₂/n₁); where n₁ is arefractive index of the semiconductor, and n₂ is a refractive index ofthe terminating substance). For example, when a polyimide of having arefractive index of 1.55 is used, φ may be smaller than 61.1 degrees.

In the present embodiment, the p-InP layer 113 at the surface side isformed by a crystal growth, however, alternatively, an undoped InP layermay be formed in crystal growth, and the conduction type of the mainpart at the surface side be determined by Zn diffusion or an ionimplantation method and subsequent anneal. Further, as the semiconductorphoto-detector, on a semiconductor layer having a first conduction type,a multilayer structure having a large schottky-barrier height opposing aschottky electrode which has a schottky-barrier higher than the schottkybarrier between the photo-absorption layer and the schottky electrodemay be constructed on the substrate, between the photo-absorption layercomprising an intrinsic or first conduction type semiconductor layer, asuperlattice semiconductor layer, or a multiple quantum wellsemiconductor layer and the schottky electrode, and a semiconductorphoto-detector may be constructed with the semiconductor layer of largeschottky barrier height comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1,0≦y≦1) or In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon. Yetfurther, the present embodiment is an example using semi-insulating InPas the substrate and an n-InP layer at the substrate side, however, ap-InP layer can be used by reversing the above p and n, and an n-InP orp-InP substrate can also be used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 10

FIG. 19 is a diagram for describing a tenth embodiment of the presentinvention. A sectional perspective diagram of photo-absorption region isthe same as in FIG. 18. Numeral 111 indicates a light incident facet,112 is a 0.2 μm thick p⁺-InGaAsP (1.2 μm composition) layer, 113 is a 1μm thick p-InP layer, 114 is a 0.5 μm thick InGaAs photo-absorptionlayer, 115 is a 1 μm thick n-InP layer, 116 is a semi-insulating InPsubstrate, 117 is a p electrode, 118 is an n electrode, 119 is aV-shaped groove, 120 is polyimide, and 121 is a single mode opticalfiber. A lead electrode and a pad electrode are omitted in this figurefor simplicity since these parts are complicated in structure andintricate to be described. The device has a photo-absorption layer areaof 30 μm×70 μm. The light incident facet 111 and the V-shaped groove 119were simultaneously formed by wet etching with bromine-methanol using asilicon nitride film mask having a T-shaped window. At this moment, thelight incident facet 111 and the V-shaped groove 119 were formedutilizing the property that the (111)A plane is formed in reverse-mesashape as shown in the figure and the (−1-11) and (111) planesperpendicular, thereto are formed in forward mesa shape in wet etchingof a (001) surface wafer with bromine-methanol. In this case, the Vshape is fabricated with good precision in the plane direction and,therefore, the depth can also be controlled with good controllability bythe window width of the mask. Naturally, the reverse-mesa part and theV-shaped groove part may be formed using another wet etching liquid or adry etching method, or utilizing other crystal plane, or utilizingadhesion of the etching mask to control the angle. Further, the V-shapedgroove part can be formed in a U shape or the like by controlling theetching condition and the mask shape.

The single mode fiber 121 is guided by the V-shaped groove 119 anddisposed opposite to the light incident facet 111 as shown in thefigure, and the space in between is buried with polyimide having arefractive index of 1.7. Further, an anti-reflection coating film isformed on the photo-detector and the fiber end surface.

Right above the main reaching area of refracted light is terminated withair, and the p electrode 117 is formed in outer peripheral part of thearea. Incident light of wavelength 1.55 μm from the lateral side isrefracted at the light incident facet and transmits at an angle ofφ=17.1 degrees as shown in the figure relative to the upper surface. Inthis case, if the refractive index of the semiconductor is assumed as3.17, total reflection occurs when φ is smaller than 71.6 degrees, andthe present embodiment satisfies this condition. When light ofwavelength 1.55 μm was introduced by the single mode fiber, a largeresponsivity value of more than 1.0 A/W was obtained at an appliedreverse bias of 1.0V. In the case of prior art structure in which the pelectrode is present on almost the entire surface of the upper surface,a value of only about 0.8 A/W was obtained.

Here, since the light incident position can be almost determined by theprecision of the mask for forming the V-shaped groove guide, withoutpositioning by mechanical movement of the fiber, high precisionpositioning was possible. Since, even when a tapered fiber is used inplace of the single mode fiber to decrease the beam size, thephoto-absorption part of the device can be small-sized to the same levelas the focus beam size. Therefore, a device capable of making ultrafastresponse by size reduction can be realized with a high-coupling statewith the fiber. With a device of a photo-absorption area of 10 μm×20 μm,high speed operation of more than a 3 dB bandwidth of 40 GHz waspossible while maintaining high responsivity. Since, as shown above, thephoto-absorption part and the fiber can be coupled with high precisionwithout a lens system, a module could be easily fabricated.

In the present embodiment, the p electrode is formed in a ring shape,however, it is needless to say that although the overall reflectivity isslightly decreased, it may be formed in any structure such as areticulated electrode shape, or the upper InP/InGaAsP layer remained asa high-concentration doped layer to decrease the sheet resistancesufficiently, or the p electrode be formed in only one side part. In thepresent embodiment, right above the reaching area of incident light isterminated with air, however, it may be any of inorganic substances suchas SiO₂ or SiNx or organic substances such as polyimide or epoxy, whichhas an appropriately smaller refractive index than the semiconductor andsatisfies the total reflection condition (φ<cos⁻¹(n₂/n₁); where n₁ is arefractive index of the semiconductor, and n₂ is a refractive index ofthe terminating substance). For example, when a polyimide of having arefractive index of 1.55 is used, φ may be smaller than 60.7 degrees.

FIG. 16 shows, as described above, a calculation result in InP ofextension factor to absorption layer thickness of effective absorptionlength relative to refractive index (n₁) of the substance between thedevice and fiber when light of wavelength 1.55 μm is incident at a mesaangle of 55 degrees. The extension factor can be substantially increasedby using a material having a large n₁. In the present embodiment,polyimide having a refractive index of 1.7 is used, the extension factorin this case provides a 41% increase relative to air, as a result, theabove increase in responsivity is obtained. It is needless to say thatthe substance between the device and fiber may be any one which has agood transmissivity to incident light and a relative index of more than1, selected from organic substances such as various polyimides includingfluorinated polyimides, various epoxies including epoxy resins,fluorinated epoxies, fluorinated epoxy acrylate resins, acrylics,metamorphic silicone resins, and inorganic substances such aschalcogenide glass having a refractive index of more than 2, or a liquidsubstance such as oil. By appropriately selecting such varioussubstances, it is possible to change the refraction angle at the lightincident facet of the photo-detector, even when using a refraction typesemiconductor photo-detector cut from the same wafer having the samelayer structure and the same mesa angle construction, and adjustment ofresponsivity according to the application is possible.

Further, as to the optical fiber, in the present embodiment, a singlemode optical fiber or a tapered fiber is used, however, it is needlessto say that one which is based on various organic substances such as aplastic fiber may be used.

In the present embodiment, the p-InP layer at the surface side is formedby a crystal growth, however, alternatively, an undoped InP layer may beformed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal.

Further, as the semiconductor photo-detector, on a semiconductor layerhaving a first conduction type, a multilayer structure having a largeschottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon. Yetfurther, the present embodiment is an example using semi-insulating InPas the substrate and an n-InP layer at the substrate side, however, ap-InP layer can be used by reversing the above p and n, and an n-InP orp-InP substrate can also be used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 11

A refraction type semiconductor photo-detector according to an eleventhembodiment of the present invention is shown in FIG. 20.

As shown in the figure, on a semi-insulating InP substrate 135, asemiconductor multilayer structure as a photo-absorption part comprisinga 1 μm thick n-InP layer 134, a 1 μm thick InGaAs photo-absorption layer133, and a 1 μm thick p-InP layer 132 is stacked, and a p electrode 136and an n electrode 137 are formed, further, a light incident facet 131inwardly angled as it separates from the surface side, so that incidentlight transits the photo-absorption layer 133 diagonally with respect tothe layer thickness direction, is formed, still further, a V-shapedgroove 138 is formed opposing the light incident facet 131. The devicehas a photo-absorption layer area of 30 μm×50 μm.

The light incident facet 131 and its vicinity are buried in aphotoresist 139 which is an organic substance.

The light incident facet 131 and the V-shaped groove 138 weresimultaneously formed by wet etching with bromine-methanol using asilicon nitride film mask having a T-shaped window.

At this moment, the light incident facet 131 and the V-shaped groove 138were formed utilizing the property that the (111)A plane is formed inreverse-mesa shape as shown in the figure and the (−1-11) and (111)planes perpendicular thereto formed in forward mesa shape in wet etchingof a (001) surface wafer with bromine-methanol.

In this case, the V shape is fabricated with good precision in the planedirection and, therefore, the depth can also be controlled with goodcontrollability by the window width of the mask.

Naturally, the reverse-mesa part and the V-shaped groove 138 may beformed using another wet etching liquid or a dry etching method, orutilizing other crystal plane, or utilizing adhesion of the etching maskto control the angle.

Since the V-shaped groove 138 is automatically formed by the fact thatthe right and left forward mesa surfaces go down with the passage ofetching time until base lines of both sides are in line with each other,the depth can also be controlled with good controllability by the maskwindow width, however, before both base lines fall in line, the grooveis in a so-called U shape.

Therefore, by controlling the etching depth with time, a U-shaped groovecan also be formed as necessary.

Further, U-shaped grooves of different shapes can be formed by changingto another etching liquid in the course of etching.

When, after forming an anti-reflection coating film on the lightincident facet 131, the entire surface is coated with a photoresist andexposed by a exposure apparatus, since the reverse-mesa visor part isnot exposed, the resist 139 as an organic substance is remained in aforward mesa shape from the visor part as shown in the figure by theresist phenomenon.

The forward mesa shape of the resist 139 can be controlled bycontrolling the exposure time or exposing the substrate diagonally.

In the present embodiment, the buried part of the resist 139 is formedby exposing the entire surface, however, alternatively, exposure may beperformed using a mask to form the buried part of the resist 139 apartby a finite length from the tip of the light incident facet 131.

Even when the single mode fiber (not shown) is put on the V-shapedgroove 138 to be guided to contact against, since the front edge of thefiber is stopped by the resist 139, the fiber tip will not hit againstthe light incident facet 131 of the device.

When, after the single mode fiber is fixed with epoxy or the like, onlythe resist part is selectively removed using an organic solvent, aphoto-detector module as a photo-detection device can be fabricated.

Here, the light incident position in horizontal and vertical directionscan be almost determined by precision of the mask for forming theV-shaped groove 138, and positioning in the optical axis direction isalso possible by butting, high precision positioning was possiblewithout positioning by delicate mechanical movement of the fiber.

For example, when light of wavelength 1.3 μm is introduced, a largeresponsivity value of more than 0.8 A/W was obtained at an appliedreverse bias of 1.5V.

Further, since, even when a tapered fiber is used in place of the singlemode fiber to decrease the beam size, high precision positioning ispossible, the photo-absorption part of the device can be small-sized tothe same level as the focus beam size.

Therefore, a device capable of making ultrafast response by sizereduction can be realized with a high-coupling state with the fiber, andeasy modular construction be achieved.

For example, with a device of a photo-absorption area of 10 μm×20 μm,fabrication of a module capable of making high speed operation of a 3 dBbandwidth of 40 GHz was possible while maintaining high responsivity.

In the present embodiment, the p-InP layer at the surface side is formedby crystal growth, however, alternatively, an undoped InP may be formedin crystal growth, and the conduction type of the main part at thesurface side be determined by Zn diffusion or an ion implantation methodand subsequent anneal.

Further, as the semiconductor photo-detector, on a semiconductor layerhaving a first conduction type, a multilayer structure having a largeschottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using semi-insulatingInP as the substrate and an n-InP layer at the substrate side, however,a p-InP layer can be used by reversing the above p and n, and an n-InPor p-InP substrate can also be used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 12

A refraction type semiconductor photo-detector according to a twelfthembodiment of the present invention is shown in FIG. 21.

As shown in the figure, on a semi-insulating InP substrate 145, asemiconductor multilayer structure as a photo-absorption part comprisinga 1 μm thick n-InP layer 144, a 1 μm thick InGaAs photo-absorption layer143, and a 1 μm thick p-InP layer 142 is stacked, and a p electrode 146and an n electrode 147 are formed, further, a light incident facet 141inwardly angled as it separates from the surface side, so that incidentlight transits the photo-absorption layer 143 diagonally with respect tothe layer thickness direction, is formed, still further, a V-shapedgroove 148 is formed opposing the light incident facet 141. The devicehas a photo-absorption layer area of 30 μm×70 μm.

The light incident facet 141 and its vicinity are buried in polyimide149 which is an organic substance.

The light incident facet 141 and the V-shaped groove 148 weresimultaneously formed by wet etching with bromine-methanol using asilicon nitride film mask having a T-shaped window.

At this moment, the light incident facet 141 and the V-shaped groove 148were formed utilizing the property that the (111)A plane is formed inreverse-mesa shape as shown in the figure and the (−1-11) and (111)planes perpendicular thereto are formed in forward mesa shape in wetetching of a (001) surface wafer with bromine-methanol.

In this case, the V shape is fabricated with good precision in the planedirection and, therefore, the depth can also be controlled with goodcontrollability by the window width of the mask.

Naturally, the reverse-mesa part and the V-shaped groove 148 may beformed using another wet etching liquid or a dry etching method, orutilizing other crystal plane, or utilizing adhesion of the etching maskto control the angle.

Since the V-shaped groove 148 is automatically formed by the fact thatthe right and left forward mesa surfaces go down with the passage ofetching time until base lines of both sides are in line with each other,the depth can also be controlled with good controllability by the maskwindow width, however, before both base lines fall in line, the grooveis in a so-called U shape.

Therefore, by controlling the etching depth with time, a U-shaped groovecan also be formed as necessary.

Further, U-shaped grooves of different shapes can be formed by changingto another etching liquid in the course of etching.

After forming an anti-reflection coating film on the light incidentfacet 141, polyimide 149 was coated on the entire surface by spincoating, and a resist coated thereon, exposure performed using a maskfor covering the light incident facet 141 apart by a finite length fromthe tip part of the light incident facet 141, and development performed.

In this case, by using a substance which is etched with a resistdeveloper as the polyimide, the polyimide 149 as an organic substance isremained in a mesa shape to the position apart by a finite length fromthe visor part as shown in the figure.

FIG. 21 is a schematic diagram showing the state after the resist isremoved using acetone.

Even when the single mode fiber (not shown) is put on the V-shapedgroove 148 to be guided to contact against, since the tip part edge ofthe fiber is stopped by the polyimide 149, the fiber tip will not hitagainst the light incident facet 141 of the device.

After butting of the single mode fiber, it was fixed including betweenthe device and the fiber with polyimide (not shown) to fabricate aphoto-absorption module as a photo-detection device.

Here, the light incident position in horizontal and vertical directionscan be almost determined by precision of the mask for forming theV-shaped groove 148, and positioning in the optical axis direction isalso possible by butting, high precision positioning was possiblewithout positioning by delicate mechanical movement of the fiber.

For example, when light of wavelength 1.3 μm is introduced, a largeresponsivity value of more than 0.8 A/W was obtained at an appliedreverse bias of 1.5V.

Further, since, also when a tapered fiber is used in place of the singlemode fiber to decrease the beam size, high precision positioning ispossible, the photo-absorption part of the device can be small-sized tothe same level as the focus beam size.

Therefore, a device capable of making ultrafast response by sizereduction can be realized with a high-coupling state with the fiber, andeasy modular construction be achieved.

For example, with a device of a photo-absorption area of 10 μm×20 μm,fabrication of a module capable of making high speed operation of a 3 dBbandwidth of 40 GHz was possible while maintaining high responsivity.

In the present embodiment, polyimide 149 is patterned using a resist asa mask and developing with a resist developer, however, patterning ofthe polyimide 149 may be performed by forming a mask such as SiO₂ or ametal and other techniques such as reactive ion etching (RIE) by oxygenor the like or reactive ion beam etching (RIBE).

Further, a photo-detection device may be fabricated by coating anotherorganic substance other than polyimide such as epoxy or the likefollowed by similar patterning, and fixing including between the deviceand fiber with the same substance.

In the present embodiment, the p-InP layer at the surface side is formedby crystal growth, however, alternatively, an undoped InP layer may beformed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using semi-insulatingInP as the substrate and an n-InP layer at the substrate side, however,a p-InP layer can be used by reversing the above p and n, and an n-InPor p-InP substrate can also be used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, a material system other than InGaAsP/InP such asInGaAlAs/InGaAsP or AlGaAs/GaAs system or a material system including astrain may be used.

Embodiment 13

FIG. 22 is a diagram for describing a thirteenth embodiment of thepresent invention. FIG. 23 is a sectional perspective diagram of thephoto-absorption region. Numeral 191 indicates a light incident facet,192 is a 0.2 μm thick p⁺-InGaAsP (1.2 μm composition) layer, 193 is a 1μm thick p-InP layer, 194 is a 0.5 μm thick InGaAs photo-absorptionlayer, 195 is a 1 μm thick n-InP layer, 196 is a semi-insulating InPsubstrate, 197 is a p electrode, 198 is an n electrode, 199 is aV-shaped groove, and 200 is a photoresist. A lead electrode and a padelectrode are omitted in this figure for simplicity since these partsare complicated in structure and intricate to be described. The devicehas a photo-absorption layer area of 30 μm×50 μm.

The light incident facet 191 and the V-shaped groove 199 weresimultaneously formed by wet etching with bromine-methanol using asilicon nitride film mask having a T-shaped window. At this moment, thelight incident facet 191 and the V-shaped groove 199 were formedutilizing the property that the (111)A plane is formed in reverse-mesashape as shown in the figure and the (−1-11) and (111) planesperpendicular thereto are formed in forward mesa shape in wet etching ofa (001) surface wafer with bromine-methanol. In this case, the V shapeis fabricated with good precision in the plane direction and, therefore,the depth can also be controlled with good controllability by the windowwidth of the mask. Naturally, the reverse-mesa part and the V-shapedgroove part may be formed using another wet etching liquid or a dryetching method, or utilizing other crystal plane, or utilizing adhesionof the etching mask to control the angle. Since the V-shaped groove 199is automatically formed by the fact that the right and left forward mesabottom sides go down with the passage of etching time until base linesof both sides are in line with each other, the depth can also becontrolled with good controllability by the mask window width, however,before both base sides fall in line, the groove is in a so-called Ushape. Therefore, by controlling the etching depth with time, a U-shapedgroove can also be formed as necessary. Further, U-shaped grooves ofdifferent shapes can be formed by changing to another etching liquid inthe course of etching.

An anti-reflection coating film is formed on the light incident facet.Right above the main reaching area of refracted incident light isterminated with air, and the p electrode 197 is formed in outerperipheral part of the area. Incident light of wavelength 1.3 μm fromthe lateral side is refracted at the light incident facet and transitsat an angle of φ=24.7 degrees as shown in the figure relative to theupper surface. In this case, if the refractive index of thesemiconductor is assumed as 3.209, total reflection occurs when φ issmaller than 71.8 degrees, and the present embodiment satisfies thiscondition. In the present embodiment, the p electrode is formed in aring shape, however, it is needless to say that although the overallreflectivity is slightly decreased, it may be formed in any structuresuch as a reticulated electrode shape, or the upper InP/InGaAsP layerremained as a high-concentration p doped layer to decrease the sheetresistance sufficiently, or the p electrode be formed in only one sidepart. In the present embodiment, right above the reaching area ofincident light is terminated with air, however, it may be any ofinorganic substances such as SiO₂ or SiNx or organic substances such aspolyimide or epoxy, which has an appropriately smaller refractive indexthan the semiconductor and satisfies the total reflection condition(φ<cos⁻¹(n₂/n₁); where n₁ is a refractive index of the semiconductor,and n₂ is a refractive index of the terminating substance). For example,when a polyimide of having a refractive index of 1.55 is used, φ may besmaller than 61.1 degrees.

When a photoresist was coated on the entire surface, and the entiresurface exposed by an exposure apparatus, since a reverse-mesa visorpart is not exposed, the resist as an organic substance is remained in aforward mesa shape from the visor part by resist development. During theexposure, the forward mesa shape of the resist can be controlled bycontrolling the exposure time or exposing the substrate diagonally. Inthe present embodiment, the buried part of the resist is formed byexposing the entire surface, however, alternatively, exposure may beperformed using a mask to form the buried part of the resist apart by afinite length from the tip part of the light incident facet. Even whenthe single mode fiber (not shown) is guided by the V-shaped groove partto contact against the photo-detector, since the tip part edge of thefiber is stopped by the resist part, the fiber tip will not hit againstthe light incident facet of the device. After fixing the fiber withepoxy or the like, only the resist part was selectively removed using anorganic solvent to fabricate a photo-absorption module as aphoto-detection device. Here, since the light incident position inhorizontal and vertical directions can be almost determined by precisionof the mask for forming the V-shaped groove guide, and positioning inthe optical axis direction is also possible by butting, high precisionpositioning was possible without positioning by delicate mechanicalmovement of the fiber. When light of wavelength 1.3 μm was introduced, alarge responsivity value of more than 0.9 A/W was obtained at an appliedreverse bias of 1.5 V.

Further, since, when a tapered fiber is used in place of the single modefiber to decrease the beam size, high precision positioning is similarlypossible, the photo-absorption part of the device can be small-sized tothe same level as the focus beam size. Therefore, a device capable ofmaking ultrafast response by size reduction can be realized with ahigh-coupling state with the fiber, and easy modular construction beachieved.

For example, with a device of a photo-absorption area of 10 μm×20 μm,fabrication of a module capable of making high speed operation of a 3 dBbandwidth of 40 GHz was possible while maintaining high responsivity. Inthe present embodiment, the p-InP layer at the surface side is formed bycrystal growth, however, alternatively, an undoped layer may be formedin crystal growth, and the conduction type of the main part at thesurface side be determined by Zn diffusion or an ion implantation methodand subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using semi-insulatingInP as the substrate and an n-InP layer at the substrate side, however,a p-InP layer can be used by reversing the above p and n, and an n-InPor p-InP substrate can also be used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 14

FIG. 24 is a diagram for describing a fourteenth embodiment of thepresent invention. A sectional perspective diagram of photo-absorptionregion is the same as in FIG. 23. Numeral 191 indicates a light incidentfacet, 192 is a 0.2 μm thick p⁺-InGaAsP (1.2 μm composition) layer, 193is a 1 μm thick p-InP layer, 194 is a 0.5 μm thick InGaAsphoto-absorption layer, 195 is a 1 μm thick n-InP layer, 196 is asemi-insulating InP substrate, 197 is a p electrode, 198 is an nelectrode, 199 is a V-shaped groove, and 201 is polyimide. A leadelectrode and a pad electrode are omitted in this figure for simplicitysince these parts are complicated in structure and intricate to bedescribed. The device has a photo-absorption layer area of 30 μm×70 μm.

The light incident facet and the V-shaped groove were simultaneouslyformed by wet etching with bromine-methanol using a silicon nitride filmmask having a T-shaped window. At this moment, the light incident facet191 and the V-shaped groove 199 were formed utilizing the property thatthe (111)A plane is formed in reverse-mesa shape as shown in the figureand the (−1-11) and (111) planes perpendicular thereto are formed inforward mesa shape in wet etching of a (001) surface wafer withbromine-methanol. In this case, the V shape is fabricated with goodprecision in the plane direction and, therefore, the depth can also becontrolled with good controllability by the window width of the mask.Naturally, the reverse-mesa part and the V-shaped groove part may beformed using another wet etching liquid or a dry etching method, orutilizing other crystal plane, or utilizing adhesion of the etching maskto control the angle. Since the V-shaped groove 199 is automaticallyformed by the fact that the right and left forward mesa bottom sides godown with the passage of etching time until base lines of both sides arein line with each other, the depth can also be controlled with goodcontrollability by the mask window width, however, before both basesides fall in line, the groove is in a so-called U shape. Therefore, bycontrolling the etching depth with time, a U-shaped groove can also beformed as necessary. Further, U-shaped grooves of different shapes canbe formed by changing to another etching liquid in the course ofetching.

In finished state of the photo-detector, the space in between thephoto-detector and the optical fiber is buried in with polyimide havinga refractive index of 1.7.

An anti-reflection coating film is formed on the light incident facet.Right above the main reaching area of refracted incident light isterminated with air, and the p electrode is formed in outer peripheralpart of the area. Incident light of wavelength 1.3 μm from the lateralside is refracted at the light incident facet and transits at an angleof φ=17.3 degrees relative to the upper surface as shown in the figurein the case that the space in between the photo-detector and the opticalfiber is buried in with polyimide. In this case, if the refractive indexof the semiconductor is assumed as 3.209, total reflection occurs when φis smaller than 71.8 degrees, and the present embodiment satisfies thiscondition. In the present embodiment, the p electrode is formed in aring shape, however, it is needless to say that although the overallreflectivity is slightly decreased, it may be formed in any structuresuch as a reticulated electrode shape, or the upper InP/InGaAsP layer beremained as a high-concentration p doped layer to sufficiently decreasethe sheet resistance, or the p electrode be formed in only one sidepart. In the present embodiment, right above the reaching area ofincident light is terminated with air, however, it may be any ofinorganic substances such as SiO₂ or SiNx or organic substances such aspolyimide or epoxy, which has an appropriately smaller refractive indexthan the semiconductor and satisfies the total reflection condition(φ<cos⁻¹(n₂/n₁); where n₁ is a refractive index of the semiconductor,and n₂ is a refractive index of the terminating substance). For example,when a polyimide of having a refractive index of 1.55 is used, φ may besmaller than 61.1 degrees.

After forming an anti-reflection coating film on the light incidentfacet, polyimide was coated on the entire surface by spin coating, and aresist coated thereon, exposure performed using a mask for covering thelight incident facet apart by a finite length from the tip part of thelight incident facet, and development performed. In this case, by usinga substance which is etched with a resist developer as the polyimide,the polyimide as an organic substance is remained in a mesa shape to theposition apart by a finite length from the visor part as shown in thefigure. This figure is a schematic diagram showing the state after theresist is removed using acetone.

Even when the single mode fiber is put on the V-shaped groove 199 to beguided to contact against, since the tip part of the fiber is stopped bythe polyimide 201, the fiber tip will not hit against the light incidentfacet 191 of the semiconductor device. After butting of the fiber, itwas fixed including between the device and the fiber with polyimide (notshown) to fabricate a photo-absorption module as a photo-detectiondevice. Here, the light incident position in horizontal and verticaldirections can be almost determined by precision of the mask for formingthe V-shaped groove guide, and positioning in the optical axis directionis also possible by butting, high precision positioning was possiblewithout positioning by delicate mechanical movement of the fiber. Whenlight of wavelength 1.3 μm was introduced, a large responsivity value ofmore than 0.9 A/W was obtained at an applied reverse bias of 1.5 V.Further, since, when a tapered fiber is used in place of the single modefiber to decrease the beam size, high precision positioning is similarlypossible, the photo-absorption part of the device can be small-sized tothe same level as the focus beam size. Therefore, a device capable ofmaking ultrafast response by size reduction can be realized with ahigh-coupling state with the fiber, and easy modular construction beachieved. With a device of a photo-absorption area of 10 μm×20 μm,fabrication of a module capable of making high speed operation of a 3 dBbandwidth of 40 GHz was possible while maintaining high responsivity.

In the present embodiment, polyimide is patterned using a resist as amask and developing with a resist developer, however, patterning of thepolyimide may be performed by forming a mask such as SiO₂ or a metal andother techniques such as reactive ion etching (RIE) by oxygen or thelike or reactive ion beam etching (RIBE). Further, a photo-detectiondevice may be fabricated by coating another organic substance other thanpolyimide such as epoxy or the like followed by similar patterning, andfixing including between the device and fiber with the same substance.Still further, it is needless to say that the substance between thedevice and fiber may be any one which has a good transmissivity toincident light and a relative index of more than 1, selected fromorganic substances such as various polyimides including fluorinatedpolyimides, various epoxies including epoxy resins, fluorinated epoxies,fluorinated epoxy acrylate resins, acrylics, metamorphic siliconeresins, and inorganic substances such as chalcogenide glass having arefractive index of more than 2, or a liquid substance such as oil. Byappropriately selecting such various substances, it is possible tochange the refraction angle at the light incident facet of thephoto-detector, even when using a refraction type semiconductorphoto-detector cut from the same wafer having the same layer structureand the same mesa angle construction, and adjustment of responsivityaccording to the application is possible.

In the present embodiment, the p-InP layer at the surface side is formedby crystal growth, however, alternatively, an undoped InP layer may beformed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using semi-insulatingInP as the substrate and an n-InP layer at the substrate side, however,a p-InP layer can be used by reversing the above p and n, and an n-InPor p-InP substrate can also be used for the fabrication.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 15

A fifteenth embodiment of the present invention is shown in FIG. 25.

In the figure, numeral 211 indicates a light incident facet, 212 is a 1μm thick p-InP layer, 213 is a 1 μm thick InGaAs photo-absorption layer,214 is a 1 μm thick n-InP layer, 215 is an n-InP substrate, 216 is a pelectrode, 217 is an n electrode, and 218 is a single mode opticalfiber.

The device has a photo-absorption layer area of 30 μm×50 μm.

The light incident facet 211 was formed utilizing the property that the(111)A plane is formed in reverse-mesa shape as shown in the figure inwet etching of a (001) surface wafer with bromine-methanol.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle.

After forming the reverse-mesa part, the substrate is cleaved at aposition 10 μm apart from the tip of the angled light incident facet211.

When, after forming an anti-reflection coating film on the lightincident facet, light of wavelength 1.3 μm was introduced, a largeresponsivity value of more than 0.8 A/W was obtained at an appliedreverse bias of 1.5 V.

In this case, when the single mode fiber 218 was roughly brought closeto the device, positioned in both horizontal and vertical directions,and finally the fiber end surface contacted against the device as shownin the figure, the fiber could be easily positioned.

Even when contacting against as shown in the figure, end surface of thefiber 218 will not contact the important light incident facet 211 todamage the facet 211, part of the fiber contacting the device is anouter peripheral part of clad which is not related to the part forguiding light, and there is no adverse effect on optical coupling.

Further, since, when a microlens holder with fiber provided with afocusing lens is used in place of the single mode fiber, the holder andthe device can be positioned with good precision in the optical axisdirection, high precision positioning is possible relatively easily evenwhen the beam size is decreased using a lens, the photo-absorption partof the device can be small-sized to the same level as the focus beamsize.

Therefore, a device capable of making ultrafast response by sizereduction can be realized.

With a device of a photo-absorption area of 10 μm×20 μm, high speedoperation of a 3 dB bandwidth of 40 GHz was possible while maintaininghigh responsivity.

As described above, since the photo-absorption part and the fiber can beeasily coupled, and the distance from the fiber and the lens holder andthe like to the light incident facet be determined almost precisely withprecision of cleavage, a module with reduced deviation in characteristiccould be easily fabricated.

In the present embodiment, the p-InP layer at the surface side is formedby crystal growth, however, alternatively, an undoped InP layer may beformed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 16

A sixteenth embodiment of the present invention is shown in FIG. 26.

In the figure, numeral 221 indicates a light incident facet, 222 is a 1μm thick p-InP layer, 223 is a 1 μm thick InGaAs photo-absorption layer,224 is a 1 μm thick n-InP layer, 225 is an n-InP substrate, 226 is a pelectrode, 227 is an n electrode, 228 is a single mode optical fiber,229 is a solder bump metal, and 230 is a pedestal.

The device has a photo-absorption layer area of 30 μm×50 μm.

The light incident facet 221 was formed utilizing the property that the(111)A plane is formed in reverse-mesa shape as shown in the figure inwet etching of a (001) surface wafer with bromine-methanol.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle.

After forming the reverse-mesa part, the substrate is cleaved at aposition 10 μm apart from the tip of the angled light incident facet221.

After forming an anti-reflection coating film on the light incidentfacet, in the present embodiment, the device is mounted on the pedestalas shown in the figure using a solder bump metal.

When light of wavelength 1.3 μm was introduced by the single mode fiber228, a large responsivity value of more than 0.8 A/W was obtained at anapplied reverse bias of 1.5 V.

In this case, since optical axis positioning in the vertical directioncan be adjusted in high precision by the thicknesses of the p electrode226 and the bump metal 229, the fiber can be positioned very easily byroughly bringing the single mode fiber close the device along thepedestal, performing positioning only in the horizontal direction, andfinally contacting the fiber end surface against the device as shown inthe figure.

Even when contacting against as shown in the figure, end surface of thefiber 228 will not contact the important light incident facet 221 todamage the facet 221, part of the fiber contacting the device is anouter peripheral part of clad which is not related to the part forguiding light, and there is no adverse effect on optical coupling.

Further, since, when a microlens holder with fiber provided with afocusing lens is used in place of the single mode fiber, the holder andthe device can be positioned with good precision in the optical axisdirection, high precision positioning is possible relatively easily evenwhen the beam size is decreased using a lens, the photo-absorption partof the device can be small-sized to the same level as the focus beamsize.

Still further, in this case, it is needless to say that heightadjustment in the vertical direction can be flexibly achieved byinserting an appropriate spacer between the electrode and the pedestal.

Therefore, a device capable of making ultrafast response by sizereduction can be realized.

With a device of a photo-absorption area of 10 μm×20 μm, high speedoperation of a 3 dB bandwidth of 40 GHz was possible while maintaininghigh responsivity.

As described above, since the photo-absorption part and the fiber can beeasily coupled, and the distance from the fiber and the lens holder andthe like to the light incident facet be determined almost precisely withprecision of cleavage, a module with reduced deviation in characteristiccould be easily fabricated.

In the present embodiment, the p electrode has a structure of directlycontacting the pedestal, however, it is needless to say that a leadelectrode from the p electrode may be provided to another part so thatthis part is contacted with the pedestal for performing electricalconnection and height adjustment.

In the present embodiment, the p-InP layer at the surface side is formedby crystal growth, however, alternatively, an undoped InP layer may beformed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Embodiment 17

FIG. 27 is a diagram for describing a seventeenth embodiment of thepresent invention. Numeral 241 indicates a light incident facet, 242 isa 1 μm thick p-InP layer, 243 is a 1 μm thick InGaAs photo-absorptionlayer, 244 is a 1 μm thick n-InP layer, 245 is an n-InP substrate, 246is a p electrode, 247 is an n electrode, and 248 is a single modeoptical fiber. The device has a photo-absorption layer area of 30 μm×50μm. The light incident facet was formed utilizing the property that the(111)A plane is formed in reverse-mesa shape as shown in the figure inwet etching of a (001) surface wafer with bromine-methanol.

Since, in this case, reverse-mesa etching is performed on the n-InPlayer 244 and the InP substrate 245 which are composed only of InP, auniform angled light incident facet of good flatness can be formed witha good yield. Further, an etching mask is formed at a position 8 μmapart from the photo-absorption part including the photo-absorptionlayer, and deep reverse-mesa etching of about 30 μm is performed, inthis case, side etching of about 3 μm occurs, however, thephoto-absorption part does not contact the side etching part, therefore,abnormal side etching, etching irregularity or the like caused byrelatively fast etching speed of the photo-absorption layer will notgenerate. In addition, uniform devices with equal mesa angle can befabricated.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle. Afterforming the reverse-mesa part, the substrate is cleaved at a position 10μm apart from the tip of the angled light incident facet. When, afterforming an anti-reflection coating film on the light incident facet,light of wavelength 1.3 μm was introduced by the single mode fiber, alarge responsivity value of more than 0.8 A/W was obtained at an appliedreverse bias of 1.5 V. In this case, when the single mode fiber wasroughly brought close to the device, positioned in both horizontal andvertical directions, and finally the fiber end surface contacted againstthe device as shown in the figure, the fiber could be easily positioned.Even when contacting against as shown in the figure, end surface of thefiber will not contact the important light incident facet to damage thefacet, part of the fiber contacting the device is an outer peripheralpart of clad which is not related to the part for guiding light, andthere is no adverse effect on optical coupling.

Further, since, when a microlens holder with fiber provided with afocusing lens is used in place of the single mode fiber, the holder andthe device can be positioned with good precision in the optical axisdirection, high precision positioning is possible relatively easily evenwhen the beam size is decreased using a lens, the photo-absorption partof the device can be small-sized to the same level as the focus beamsize. Therefore, a device capable of making ultrafast response by sizereduction can be realized. With a device of a photo-absorption area of10 μm×20 μm, high speed operation of a 3 dB bandwidth of 40 GHz waspossible while maintaining high responsivity. As described above, sincethe photo-absorption part and the fiber can be easily coupled, and thedistance from the fiber and the lens holder and the like to the lightincident facet be determined almost precisely with precision ofcleavage, a module with reduced deviation in characteristic could beeasily fabricated.

In the present embodiment, the p-InP layer at the surface side is formedby crystal growth, however, alternatively, an undoped InP layer may beformed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon. Yetfurther, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used. Yet further, it is needless to saythat a material system other than InGaAsP/InP such as InGaAlAs/InGaAsPor AlGaAs/GaAs system or a material system including a strain may beused.

Embodiment 18

FIG. 28 is a diagram for describing a eighteenth embodiment of thepresent invention. Numeral 251 indicates a light incident facet, 252 isa 1 μm thick p-InP layer, 253 is a 0.7 μm thick InGaAs photo-absorptionlayer, 254 is a 1 μm thick n-InP layer, 255 is an n-InP substrate, 256is a p electrode, 257 is an n electrode, 258 is a single mode opticalfiber, and 259 is polyimide. The device has a photo-absorption layerarea of 30 μm×70 μm. The light incident facet 251 was formed utilizingthe property that the (111)A plane is formed in reverse-mesa shape asshown in the figure in wet etching of a (001) surface wafer withbromine-methanol.

Since, in this case, reverse-mesa etching is performed on the n-InPlayer 254 and the InP substrate 255 which are composed only of InP, auniform angled light incident facet of good flatness can be formed witha good yield. Further, an etching mask is formed at a position 8 μmapart from the photo-absorption part including the photo-absorptionlayer, and deep reverse-mesa etching of about 30 μm is performed, inthis case, side etching of about 3 μm occurs, however, thephoto-absorption part does not contact the side etching part, therefore,abnormal side etching, etching irregularity or the like caused byrelatively fast etching speed of the photo-absorption layer will notgenerate. In addition, uniform devices with equal mesa angle can befabricated.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle. Afterforming the reverse-mesa part, the substrate is cleaved at a position 10μm apart from the tip of the angled light incident facet. When thesingle mode fiber was roughly brought close to the device, positioned inboth horizontal and vertical directions, and finally the fiber endsurface contacted against the device as shown in the figure, the fibercould be easily positioned. Even when contacting against as shown in thefigure, end surface of the fiber will not contact the important lightincident facet to damage the facet, part of the fiber contacting thedevice is an outer peripheral part of clad which is not related to thepart for guiding light, and there is no adverse effect on opticalcoupling. Further, since, when a microlens holder with fiber providedwith a focusing lens is used in place of the single mode fiber, theholder and the device can be positioned with good precision in theoptical axis direction, high precision positioning is possiblerelatively easily even when the beam size is decreased using a lens, thephoto-absorption part of the device can be small-sized to the same levelas the focus beam size. Therefore, a device capable of making ultrafastresponse by size reduction can be realized. As described above, sincethe photo-absorption part and the fiber can be easily coupled, and thedistance from the fiber and the lens holder and the like to the lightincident facet be determined almost precisely with precision ofcleavage, a module with reduced deviation in characteristic could beeasily fabricated.

Further, in the present embodiment, after the single mode fiber isdisposed opposite to the light incident facet as shown in the figure,the space in between is buried in with polyimide having a refractiveindex of 1.7. Still further, an anti-reflection coating film is formedon the photo-detector and the fiber end surface. When light ofwavelength 1.55 μm was introduced by the single mode fiber, a largeresponsivity value of 1.0 A/W was obtained at an applied reverse bias of1.5 V. The layer structure of the photo-detector of the presentembodiment is designed for enabling high speed operation. That is, thephoto-absorption layer is as thin as 0.7 μm, so that the carrier transittime is reduced. Yet further, the refraction angle is the largest whenthe medium between the device and the fiber is air, therefore, a devicelength required for receiving refracted light can be reduced, and thedevice capacity determined by the device size can be reduced. In theprior art using air between the device and fiber, with a module having areduced device size to 10 μm×20 μm, high speed operation of more than a3 dB bandwidth of 40 GHz could be confirmed. However, in this module,the responsivity was 0.8 A/W. This is because absorption in thephoto-absorption layer is insufficient. Since high speed andresponsivity are basically in a trade-off relation, a decrease inresponsivity associated with high speed is inevitable to some extent,however, according to the present invention, it is possible to changethe responsivity over a wide range even when the absorption layer isdecreased in thickness for high speed operation. However, the operationspeed is decreased to some extent by increasing the device capacityassociated with an increase in photo-detector length.

FIG. 29 shows a calculation result in InP of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of the substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees.The extension factor can be substantially increased by using a materialhaving a large n₁. In the present embodiment, polyimide having arefractive index of 1.7 is used, the extension factor in this caseprovides a 41% increase relative to air, as a result, the above increasein responsivity is obtained. It is needless to say that the substancebetween the device and fiber may be any one which has a goodtransmissivity to incident light and a relative index of more than 1,selected from organic substances such as various polyimides includingfluorinated polyimides, various epoxies including epoxy resins,fluorinated epoxies, fluorinated epoxy acrylate resins, acrylics,metamorphic silicone resins, and inorganic substances such aschalcogenide glass having a refractive index of more than 2, or a liquidsubstance such as oil. By appropriately selecting such varioussubstances, it is possible to change the refraction angle at the lightincident facet of the photo-detector, even when using a refraction typesemiconductor photo-detector cut from the same wafer having the samelayer structure and the same mesa angle construction, and adjustment ofresponsivity according to the application is possible. As to thephoto-detector, in the present embodiment, the p-InP layer at thesurface side is formed by crystal growth, however, alternatively, anundoped InP layer may be formed in crystal growth, and the conductiontype of the main part at the surface side be determined by Zn diffusionor an ion implantation method and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon. Yetfurther, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Yet further, as to the optical waveguide, a single mode optical fiber isused in the present embodiment, however, alternatively, a lensed fiber,a tapered fiber, a silica-based waveguide circuit such as PlanarLightwave Circuit (PLC) or the like may be used.

Yet further, it is needless to say that a hybrid integrated constructionof mounting a photo-detector on PLC may be used. Yet further, it isneedless to say that the waveguide is not only those based on variousinorganic materials such as silica-based circuit but may be those basedon various organic materials such as polymer waveguide or plastic fiber.

Embodiment 19

FIG. 30 is a sectional perspective diagram for describing a nineteenthembodiment of the present invention. Numeral 261 indicates a lightincident facet, 262 is a 0.2 μm thick p⁺-InGaAsP layer (1.2 μmcomposition), 263 is a 1 μm thick p-InP layer, 264 is a 0.5 μm thickInGaAs photo-absorption layer, 265 is a 1 μm thick n-InP layer, 266 is asemi-insulating InP substrate, 267 is a p electrode; 268 is an nelectrode, and 269 is a single mode optical fiber. A lead electrode anda pad electrode are omitted in this figure for simplicity since theseparts are complicated in structure and intricate to be described. Thedevice has a photo-absorption layer area of 30 μm×50 μm. The lightincident facet 261 was formed utilizing the property that the (111)Aplane is formed in reverse-mesa shape of about 55 degrees as shown inthe figure in wet etching of a (001) surface wafer withbromine-methanol.

Since, in this case, reverse-mesa etching is performed on the InP layer265 and the InP substrate 266 which are composed only of InP, a uniformangled light incident facet of good flatness can be formed with a goodyield. Further, an etching mask is formed at a position 8 μm apart fromthe photo-absorption part including the photo-absorption layer, and deepreverse-mesa etching of about 30 μm is performed, in this case, sideetching of about 3 μm occurs, however, the photo-absorption part doesnot contact the side etching part, therefore, abnormal side etching,etching irregularity or the like caused by relatively fast etching speedof the photo-absorption layer will not generate. In addition, uniformdevices with equal mesa angle can be fabricated.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle.

An anti-reflection coating film is formed on the light incident facet.Right above the main reaching area of refracted incident light isterminated with air, and the p electrode is formed in outer peripheralpart of the area. Incident light of wavelength 1.55 μm from the lateralside is refracted at the light incident facet and transits at an angleof φ=17.1 degrees relative to the upper surface as shown in the figure.In this case, if the refractive index of the semiconductor is assumed as3.17, total reflection occurs when φ is smaller than 71.6 degrees, andthe present embodiment satisfies this condition.

In the present embodiment, the p electrode is formed in a ring shape,however, it is needless to say that although the overall reflectivity isslightly decreased, it may be formed in any structure such as areticulated electrode shape, or the upper InP/InGaAsP layer be remainedas a high-concentration p doped layer to sufficiently decrease the sheetresistance, or the p electrode be formed in only one side part. In thepresent embodiment, right above the reaching area of incident light isterminated with air, however, it may be any of inorganic substances suchas SiO₂ or SiNx or organic substances such as polyimide or epoxy, whichhas an appropriately smaller refractive index than the semiconductor andsatisfies the total reflection condition (φ<cos⁻¹(n₂/n₁); where n₁ is arefractive index of the semiconductor, and n₂ is a refractive index ofthe terminating substance). For example, when a polyimide of having arefractive index of 1.55 is used, φ may be smaller than 60.7 degrees.

After forming the reverse-mesa part, the substrate is cleaved at aposition 10 μm apart from the tip of the angled light incident facet.When the single mode fiber 269 was roughly brought close to the device,positioned in both horizontal and vertical directions, and finally thefiber end surface contacted against the device as shown in the figure,the fiber could be easily positioned. Even when contacting against asshown in the figure, end surface of the fiber will not contact theimportant light incident facet to damage the facet, part of the fibercontacting the device is an outer peripheral part of clad which is notrelated to the part for guiding light, and there is no adverse effect onoptical coupling. Further, since, when a microlens holder with fiberprovided with a focusing lens is used in place of the single mode fiber,the holder and the device can be positioned with good precision in theoptical axis direction, high precision positioning is possiblerelatively easily even when the beam size is decreased using a lens, thephoto-absorption part of the device can be small-sized to the same levelas the focus beam size. Therefore, a device capable of making ultrafastresponse by size reduction can be realized. As described above, sincethe photo-absorption part and the fiber can be easily coupled, and thedistance from the fiber and the lens holder and the like to the lightincident facet be determined almost precisely with precision ofcleavage, a module with reduced deviation in characteristic could beeasily fabricated.

When light of wavelength 1.55 μm was introduced using the single modefiber 269, a large responsivity value of 0.9 A/W was obtained at anapplied reverse bias of 1.5 V. The layer structure of the photo-detectorof the present embodiment is designed for enabling high speed operation.That is, the photo-absorption layer is as thin as 0.5 μm, so that thecarrier transit time is reduced. As to the photo-detector, in thepresent embodiment, the p-InP layer at the surface side is formed bycrystal growth, however, alternatively, an undoped InP layer may beformed in crystal growth, and the conduction type of the main part atthe surface side be determined by Zn diffusion or an ion implantationmethod and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon. Yetfurther, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Yet further, as to the optical waveguide, a single mode optical fiber isused in the present embodiment, however, alternatively, a lensed fiber,a tapered fiber, a silica-based waveguide circuit such as PlanarLightwave Circuit (PLC) or the like may be used.

Yet further, it is needless to say that a hybrid integrated constructionof mounting a photo-detector on PLC may be used.

Yet further, it is needless to say that the waveguide is not only thosebased on various inorganic materials such as silica-based waveguide butmay be those based on various organic materials such as polymerwaveguide or plastic fiber.

Embodiment 20

FIG. 31 is a sectional perspective diagram for describing a twentiethembodiment of the present invention. In the figure, same subject mattersas in FIG. 30 are indicated with same reference numerals and descriptionthereof is simplified. In the figure, numeral 270 indicates polyimide(refractive index 1.7). A lead electrode and a pad electrode are omittedin this figure for simplicity since these parts are complicated instructure and intricate to be described. The device has aphoto-absorption layer area of 30 μm×70 μm. The light incident facet 261was formed utilizing the property that the (111)A plane is formed inreverse-mesa shape of about 55 degrees as shown in the figure in wetetching of a (001) surface wafer with bromine-methanol.

Since, in this case, reverse-mesa etching is performed on the InP layer265 and the InP substrate 266 which are composed only of InP, a uniformangled light incident facet 261 of good flatness can be formed with agood yield. Further, an etching mask is formed at a position 8 μm apartfrom the photo-absorption part including the photo-absorption layer, anddeep reverse-mesa etching of about 30 μm is performed, in this case,side etching of about 3 μm occurs, however, the photo-absorption partdoes not contact the side etching part, therefore, abnormal sideetching, etching irregularity or the like caused by relatively fastetching speed of the photo-absorption layer will not generate. Inaddition, uniform devices with equal mesa angle can be fabricated.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle.

An anti-reflection coating film is formed on the light incident facet.Right above the main reaching area of refracted incident light isterminated with air, and the p electrode 267 is formed in outerperipheral part of the area. Incident light of wavelength 1.55 μm fromthe lateral side is refracted at the light incident facet 261 andtransits at an angle of φ=17.1 degrees relative to the upper surface asshown in the figure. In this case, if the refractive index of thesemiconductor is assumed as 3.17, total reflection occurs when φ issmaller than 71.6 degrees, and the present embodiment satisfies thiscondition. In the present embodiment, the p electrode is formed in aring shape, however, it is needless to say that although the overallreflectivity is slightly decreased, it may be formed in any structuresuch as a reticulated electrode shape, or the upper InP/InGaAsP layer beremained as a high-concentration p doped layer to sufficiently decreasethe sheet resistance, or the p electrode be formed in only one sidepart. In the present embodiment, right above the reaching area ofincident light is terminated with air, however, it may be any ofinorganic substances such as SiO₂ or SiNx or organic substances such aspolyimide or epoxy, which has an appropriately smaller refractive indexthan the semiconductor and satisfies the total reflection condition(φ<cos⁻¹(n₂/n₁); where n₁ is a refractive index of the semiconductor,and n₂ is a refractive index of the terminating substance). For example,when a polyimide of having a refractive index of 1.55 is used, φ may besmaller than 60.7 degrees.

After forming the reverse-mesa part, the substrate is cleaved at aposition 10 μm apart from the tip of the angled light incident facet.When the single mode fiber 269 was roughly brought close to the device,positioned in both horizontal and vertical directions, and finally thefiber end surface contacted against the device as shown in the figure,the fiber could be easily positioned. Even when contacting against asshown in the figure, end surface of the fiber will not contact theimportant light incident facet to damage the facet, part of the fibercontacting the device is an outer peripheral part of clad which is notrelated to the part for guiding light, and there is no adverse effect onoptical coupling. Further, since, when a microlens holder with fiberprovided with a focusing lens is used in place of the single mode fiber,the holder and the device can be positioned with good precision in theoptical axis direction, high precision positioning is possiblerelatively easily even when the beam size is decreased using a lens, thephoto-absorption part of the device can be small-sized to the same levelas the focus beam size. Therefore, a device capable of making ultrafastresponse by size reduction can be realized. As described above, sincethe photo-absorption part and the fiber can be easily coupled, and thedistance from the fiber and the lens holder and the like to the lightincident facet be determined almost precisely with precision ofcleavage, a module with reduced deviation in characteristic could beeasily fabricated.

Further, in the present embodiment, after the single mode fiber isdisposed opposite to the light incident facet as shown in the figure,the space in between is buried in with polyimide having a refractiveindex of 1.7. Still further, an anti-reflection coating film is formedon the photo-detector and the fiber end surface. When light ofwavelength 1.55 μm was introduced by the single mode fiber, a largeresponsivity value of 1.0 A/W was obtained at an applied reverse bias of1.5 V. The layer structure of the photo-detector of the presentembodiment is designed for enabling high speed operation. That is, thephoto-absorption layer is as thin as 0.5 μm, so that the carrier transittime is reduced.

Yet further, the refraction angle is the largest when the medium betweenthe device and the fiber is air, therefore, a device length required forreceiving refracted light can be reduced, and the device capacitydetermined by the device size can be reduced. In the prior art using airbetween the device and fiber, with a module having a reduced device sizeto 10 μm×20 μm, high speed operation of more than a 3 dB bandwidth of 40GHz could be confirmed. However, in this module, the responsivity was0.8 A/W. This is because absorption in the photo-absorption layer isinsufficient. Since high speed and responsivity are basically in atrade-off relation, a decrease in responsivity associated with highspeed is inevitable to some extent, however, according to the presentinvention, it is possible to change the responsivity over a wide rangeeven when the absorption layer is decreased in thickness for high speedoperation. However, the operation speed is decreased to some extent byincreasing the device capacitance associated with an increase inphoto-detector length.

FIG. 29 shows a calculation result in InP of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of the substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees.The extension factor can be substantially increased by using a materialhaving a large n₁. In the present embodiment, polyimide having arefractive index of 1.7 is used, the extension factor in this caseprovides a 41% increase relative to air, as a result, the above increasein responsivity is obtained. It is needless to say that the substancebetween the device and fiber may be any one which has a goodtransmissivity to incident light and a relative index of more than 1,selected from organic substances such as various polyimides includingfluorinated polyimides, various epoxies including epoxy resins,fluorinated epoxies, fluorinated epoxy acrylate resins, acrylics,metamorphic silicone resins, and inorganic substances such aschalcogenide glass having a refractive index of more than 2, or a liquidsubstance such as oil. By appropriately selecting such varioussubstances, it is possible to change the refraction angle at the lightincident facet of the photo-detector, even when using a refraction typesemiconductor photo-detector cut from the same wafer having the samelayer structure and the same mesa angle construction, and adjustment ofresponsivity according to the application is possible.

As to the photo-detector, in the present embodiment, the p-InP layer atthe surface side is formed by crystal growth, however, alternatively, anundoped InP layer may be formed in crystal growth, and the conductiontype of the main part at the surface side be determined by Zn diffusionor an ion implantation method and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon. Yetfurther, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Yet further, as to the optical waveguide, a single mode optical fiber isused in the present embodiment, however, alternatively, a lensed fiber,a tapered fiber, a silica-based waveguide circuit such as PlanarLightwave Circuit (PLC) or the like may be used.

Yet further, it is needless to say that a hybrid integrated constructionof mounting a photo-detector on PLC may be used.

Yet further, it is needless to say that the waveguide is not only thosebased on various inorganic materials such as silica-based waveguide butmay be those based on various organic materials such as polymerwaveguide or plastic fiber.

Embodiment 21

A semiconductor photo-detector according to a twenty-first embodiment ofthe present invention is shown in FIG. 32.

In FIG. 32, a lead electrode and a pad electrode are omitted forsimplicity since these parts are complicated in structure and intricateto be described.

This semiconductor photo-detector is a refraction type semiconductorphoto-detector in which a photo-absorption part comprising a 1 μm thickn-InP layer 285, a 0.5 μm thick InGaAs photo-absorption layer 284, and a1 μm thick p-InP layer 283 is successively stacked in this order on asemi-insulating InP substrate 286, and a light incident facet 281 whichis inwardly angled as it separates from the surface side is provided onan end surface, so that incident light is refracted at the lightincident facet 281 and transits the photo-absorption layer 284diagonally with respect to the layer thickness direction. The device hasa photo-absorption layer area of 30 μm×50 μm.

The light incident facet 281 was formed utilizing the property that the(111)A plane is formed in reverse-mesa shape of about 55 degrees asshown in the figure in wet etching of a (001) surface wafer withbromine-methanol.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle. Ananti-reflection coating film is formed on the light incident facet 281.

Further, in the present embodiment, right above the main reaching areaof refracted incident light in the upper layer of the photo-absorptionlayer 284 is terminated with air, and the p electrode 287 is formed inouter peripheral part of the area through the 0.2 μm thick p⁺-InGaAsP(1.2 μm composition) layer 282.

Therefore, incident light of wavelength 1.3 μm from the lateral side isrefracted at the light incident facet 281 and transits at an angle ofφ=24.7 degrees relative to the upper surface as shown in the figure.

In this case, if the refractive index of the semiconductor is assumed as3.209, total reflection occurs when φ is smaller than 71.8 degrees, andthe present embodiment satisfies this condition.

When light of wavelength 1.3 μm was introduced by the single mode fiber,a large responsivity value of more than 0.9 A/W was obtained at anapplied reverse bias of 1.0 V.

In the case of prior art structure in which the p electrode is presenton almost the entire upper surface, a value of only about 0.7 A/W wasobtained.

In the present embodiment, the p electrode 287 is formed in a ringshape, however, although the overall reflectivity is slightly decreased,it may be formed in any structure such as a reticulated electrode shape,or the upper InP/InGaAsP layer be remained as a high-concentration pdoped layer to sufficiently decrease the sheet resistance, or the pelectrode be formed in only one side part.

Further, when using a tapered fiber in place of the single mode fiber toreduce the device size (photo-absorption area 7 μm×20 μm, high speedoperation of 3 dB band 50 GHz was possible while maintaining highresponsivity.

In the present embodiment, right above the reaching area of incidentlight is terminated with air, however, it may be any of inorganicsubstances such as SiO₂ or SiNx or organic substances such as polyimideor epoxy, which has an appropriately smaller refractive index than thesemiconductor and satisfies the total reflection condition(φ<cos⁻¹(n₂/n₁); where n₁ is a refractive index of the semiconductor,and n₂ is a refractive index of the terminating substance).

For example, when a polyimide having a refractive index of 1.55 is used,φ may be smaller than 61.1 degrees.

In the present embodiment, the p-InP layer 283 at the surface side isformed by crystal growth, however, alternatively, an undoped InP layermay be formed in crystal growth, and the conduction type of the mainpart at the surface side be determined by Zn diffusion or an ionimplantation method and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using semi-insulatingInP as the substrate and an n-InP layer at the substrate side, however,a p-InP layer can be used by reversing the above p and n, and an n-InPor p-InP substrate can also be used for the fabrication.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, a material system other than InGaAsP/InP such asInGaAlAs/InGaAsP or AlGaAs/GaAs system or a material system including astrain may be used.

Embodiment 22

FIG. 33 is a sectional perspective diagram for describing atwenty-second embodiment of the present invention. Numeral 291 indicatesa light incident facet, 292 is a 0.2 μm thick p⁺-InGaAsP layer (1.2 μmcomposition), 293 is a 1 μm thick p-InP layer, 294 is a 0.5 μm thickInGaAs photo-absorption layer, 295 is a 1 μm thick n-InP layer, 296 is asemi-insulating InP substrate, 297 is a p electrode, 298 is an nelectrode, 299 is a single mode optical fiber, and 300 is polyimide(refractive index 1.7). A lead electrode and a pad electrode are omittedin this figure for simplicity since these parts are complicated instructure and difficult to be described. The device has aphoto-absorption layer area of 30 μm×70 μm. The light incident facet 291was formed utilizing the property that the (111)A plane is formed inreverse-mesa shape of about 55 degrees as shown in the figure in wetetching of a (001) surface wafer with bromine-methanol.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle. Ananti-reflection coating film is formed on the light incident facet.Right above the main reaching area of refracted incident light isterminated with air, and the p electrode 297 is formed in outerperipheral part of the area. Incident light of wavelength 1.55 μm fromthe lateral side is refracted at the light incident facet and transitsat an angle of φ=17.1 degrees relative to the upper surface as shown inthe figure. In this case, if the refractive index of the semiconductoris assumed as 3.17, total reflection occurs when φ is smaller than 71.6degrees, and the present embodiment satisfies this condition.

In the present embodiment, the p electrode 297 is formed in a ringshape, however, it is needless to say that although the overallreflectivity is slightly decreased, it may be formed in any structuresuch as a reticulated electrode shape, or the upper InP/InGaAsP layer beremained as a high-concentration p doped layer to sufficiently decreasethe sheet resistance, or the p electrode be formed in only one sidepart.

In the present embodiment, right above the reaching area of incidentlight is terminated with air, however, it may be any of inorganicsubstances such as SiO₂ or SiNx or organic substances such as polyimideor epoxy, which has an appropriately smaller refractive index than thesemiconductor and satisfies the total reflection condition (φ<cos⁻¹(n₂/n₁); where n₁ is a refractive index of the semiconductor, and n₂ isa refractive index of the terminating substance). For example, when apolyimide having a refractive index of 1.55 is used, may be smaller than60.7 degrees.

Further, in the present embodiment, after the single mode fiber isdisposed opposite to the light incident facet as shown in the figure,the space in between is buried in with polyimide having a refractiveindex of 1.7. Still further, an anti-reflection coating film is formedon the photo-detector and the fiber end surface. When light ofwavelength 1.55 μm was introduced by the single mode fiber, a largeresponsivity value of 1.0 A/W was obtained at an applied reverse bias of1.5 V. The layer structure of the photo-detector of the presentembodiment is designed for enabling high speed operation. That is, thephoto-absorption layer is as thin as 0.5 μm, so that the carrier transittime is reduced. Yet further, the refraction angle is the largest whenthe medium between the device and the fiber is air, therefore, a devicelength required for receiving refracted light can be reduced, and thedevice capacity determined by the device size can be reduced. In theprior art using air between the device and fiber, with a module having areduced device size to 10 μm×20 μm, high speed operation of more than a3 dB bandwidth of 40 GHz could be confirmed. However, in this module,the responsivity was 0.8 A/W. This is because absorption in thephoto-absorption layer is insufficient. Since high speed andresponsivity are basically in a trade-off relation, a decrease inresponsivity associated with high speed is inevitable to some extent,however, according to the present invention, it is possible to changethe responsivity over a wide range even when the absorption layer isdecreased in thickness for high speed operation. However, the operationspeed is decreased to some extent by increasing the device capacitanceassociated with an increase in photo-detector length.

FIG. 34 shows a calculation result in InP of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of the substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees.The extension factor can be substantially increased by using a materialhaving a large n₁. In the present embodiment, polyimide having arefractive index of 1.7 is used, the extension factor in this caseprovides a 41% increase relative to air, as a result, the above increasein responsivity is obtained. It is needless to say that the substancebetween the device and fiber may be any one which has a goodtransmissivity to incident light and a relative index of more than 1,selected from organic substances such as various polyimides includingfluorinated polyimides, various epoxies including epoxy resins,fluorinated epoxies, fluorinated epoxy acrylate resins, acrylics,metamorphic silicone resins, and inorganic substances such aschalcogenide glass having a refractive index of more than 2, or a liquidsubstance such as oil. By appropriately selecting such varioussubstances, it is possible to change the refraction angle at the lightincident facet of the photo-detector, even when using a refraction typesemiconductor photo-detector cut from the same wafer having the samelayer structure and the same mesa angle construction, and adjustment ofresponsivity according to the application is possible.

As to the photo-detector, in the present embodiment, the p-InP layer atthe surface side is formed by crystal growth, however, alternatively, anundoped InP layer may be formed in crystal growth, and the conductiontype of the main part at the surface side be determined by Zn diffusionor an ion implantation method and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode. Yetfurther, a uniform composition bulk is used here as the photo-absorptionlayer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used. Yet further, it is needless to saythat a material system other than InGaAsP/InP such as InGaAlAs/InGaAsPor AlGaAs/GaAs system or a material system including a strain may beused.

Yet further, as to the optical waveguide, a single mode optical fiber isused in the present embodiment, however, alternatively, a lensed fiber,a tapered fiber, a silica-based waveguide circuit such as PlanarLightwave Circuit (PLC) or the like may be used.

Yet further, it is needless to say that a hybrid integrated constructionof mounting a photo-detector on PLC may be used.

Yet further, it is needless to say that the waveguide is not only thosebased on various inorganic materials such as silica-based waveguide butmay be those based on various organic materials such as polymerwaveguide or plastic fiber.

Embodiment 23

A semiconductor photo-detection device according to a twenty-thirdembodiment of the present invention is shown in FIG. 35.

In this semiconductor photo-detection device, as shown in FIG. 35, alight incident facet 311 of a refraction type semiconductorphoto-detector and a single mode optical fiber 318 are disposed oppositeto each other, and space in between is buried in with polyimide 319.

The refraction type semiconductor photo-detector has a structure inwhich a 1 μm thick p-InP layer 312, a 0.7 μm thick InGaAsphoto-absorption layer 313, and a 1 μm thick n-InP layer 314 are stackedon an n-InP substrate 315 to form a semiconductor multilayer structure,and an end surface of which is provided with an inwardly angled lightincident facet 311 so that incident light transits the photo-absorptionlayer diagonally. Further, a p electrode 316 and an n electrode 317 areprovided on upper and lower surfaces of the device. The device has aphoto-absorption layer area of 30 μm×70 μm.

The light incident facet 311 was formed utilizing the property that the(111)A plane is formed in reverse-mesa shape of about 55 degrees asshown in the figure in wet etching of a (001) surface wafer withbromine-methanol.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle.

Refractive index of polyimide 319 for filling between the single modeoptical fiber 318 and the light incident facet 311 is greater than 1,which is about 1.7.

Further, an anti-reflection coating film is formed on the photo-detectorand the fiber end surface.

When light of wavelength 1.55 μm was introduced by the single mode fiber318, a large responsivity value of 1.0 A/W was obtained at an appliedreverse bias of 1.5 V.

Still further, the layer structure of the photo-detector of the presentembodiment is designed for enabling high speed operation.

That is, the photo-absorption layer is as thin as 0.7 μm, so that thecarrier transit time is reduced.

Yet further, the refraction angle is the largest when the medium betweenthe device and the fiber is air, therefore, a device length required forreceiving refracted light can be reduced, and the device capacitancedetermined by the device size can be reduced.

In the prior art using air between the device and fiber, with a modulehaving a reduced device size to 10 μm×20 μm, high speed operation ofmore than a 3 dB bandwidth of 40 GHz could be confirmed.

However, in this module, the responsivity was 0.8 A/W. This is becauseabsorption in the photo-absorption layer 313 is insufficient.

Since high speed and responsivity are basically in a trade-off relation,a decrease in responsivity associated with high speed is inevitable tosome extent, however, according to the present invention, it is possibleto change the responsivity over a wide range even when the absorptionlayer is decreased in thickness for high speed operation.

However, the operation speed is decreased to some extent by increasingthe device capacitance associated with an increase in photo-detectorlength.

FIG. 36 shows a calculation result in InP of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of the substance between the device and fiber whenlight of wavelength 1.55 μm is incident at a mesa angle of 55 degrees.

The extension factor, as shown in FIG. 37 which is an enlarged diagramof the light incident facet part, is given by 1/sin φ as an extensionfactor of an effective absorption length teff to an absorption layerthickness t, where φ is an angle of refracted light to the absorptionlayer.

Therefore, it can be seen that the extension factor can be substantiallyincreased by using a material having a large n₁.

In the present embodiment, polyimide having a refractive index of 1.7 isused, the extension factor in this case provides a 41% increase relativeto air, as a result, the above increase in responsivity is obtained.

The substance between the device and fiber may be any one which has agood transmissivity to incident light and a relative index of more than1, selected from organic substances such as various polyimides includingfluorinated polyimides, various epoxies including epoxy resins,fluorinated epoxies, fluorinated epoxy acrylate resins, acrylics,metamorphic silicone resins, and inorganic substances such aschalcogenide glass having a refractive index of more than 2, or a liquidsubstance such as oil.

By appropriately selecting such various substances, it is possible tochange the refraction angle at the light incident facet of thephoto-detector, even when using a refraction type semiconductorphoto-detector cut from the same wafer having the same layer structureand the same mesa angle construction, and adjustment of responsivityaccording to the application is possible.

As to the photo-detector, in the present embodiment, the p-InP layer atthe surface side is formed by crystal growth, however, alternatively, anundoped InP layer may be formed in crystal growth, and the conductiontype of the main part at the surface side be determined by Zn diffusionor an ion implantation method and subsequent anneal.

Further, as the semiconductor photo-detector part, on a semiconductorlayer having a first conduction type, a multilayer structure having alarge schottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, a SAGM(Separate-absorption-graded-multiplication) structure or a SAM-SL(Separate absorption and multiplication superlattice) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, a material system other than InGaAsP/InP such asInGaAlAs/InGaAsP or AlGaAs/GaAs system or a material system including astrain may be used.

Yet further, as to the optical waveguide, a single mode optical fiber isused in the present embodiment, however, alternatively, a lensed fiber,a tapered fiber, a silica-based waveguide circuit such as PlanarLightwave Circuit (PLC) or the like may be used.

Yet further, a hybrid integrated construction of mounting aphoto-detector on PLC may be used.

Yet further, the waveguide is not only those based on various inorganicmaterials such as silica-based waveguide but may be those based onvarious organic materials such as polymer waveguide or plastic fiber.

Embodiment 24

FIG. 38 is a diagram for describing a twenty-fourth embodiment of thepresent invention. Numeral 321 indicates a light incident facet, 322 isa 1 μm thick p-InP layer, 323 is a 0.7 μm thick InGaAs layer, 324 is a 1μm thick n-InP layer, 325 is an n-InP substrate, 326 is a p electrode,327 is an n electrode, 328 is a single mode optical fiber, and 329 ispolyimide. The device has a photo-absorption layer area of 30 μm×70 μm.The light incident facet was formed utilizing the property that the(111)A plane is formed in reverse-mesa shape of about 55 degrees asshown in the figure in wet etching of a (001) surface wafer withbromine-methanol.

Since, in this case, reverse-mesa etching is performed on the n-InPlayer 324 and the InP substrate 325 which are composed only of InP, auniform angled light incident facet of good flatness can be formed witha good yield. Further, an etching mask is formed at a position 8 μmapart from the photo-absorption part including the photo-absorptionlayer, and deep reverse-mesa etching of about 30 μm is performed, inthis case, side etching of about 3 μm occurs, however, thephoto-absorption part does not contact the side etching part, therefore,abnormal side etching, etching irregularity or the like caused byrelatively fast etching speed of the photo-absorption layer will notgenerate. In addition, uniform devices with equal mesa angle can befabricated.

Naturally, the reverse-mesa part may be formed using another wet etchingliquid or a dry etching method, or utilizing other crystal plane, orutilizing adhesion of the etching mask to control the angle. After thesingle mode fiber is disposed opposite to the light incident facet asshown in the figure, the space in between is buried in with polyimidehaving a refractive index of 1.7.

Further, an anti-reflection coating film is formed on the photo-detectorand the fiber end surface. When light of wavelength 1.55 μm wasintroduced by the single mode fiber, a large responsivity value of 1.0A/W was obtained at an applied reverse bias of 1.5 V.

Still further, the layer structure of the photo-detector of the presentembodiment is designed for enabling high speed operation. That is, thephoto-absorption layer is as thin as 0.7 μm, so that the carrier transittime is reduced.

Yet further, the refraction angle is the largest when the medium betweenthe device and the fiber is air, therefore, a device length required forreceiving refracted light can be reduced, and the device capacitancedetermined by the device size can be reduced. In the prior art using airbetween the device and fiber, with a module having a reduced device sizeto 10 μm×20 μm, high speed operation of more than a 3 dB bandwidth of 40GHz could be confirmed. However, in this module, the responsivity was0.8 A/W. This is because absorption in the photo-absorption layer isinsufficient. Since high speed and responsivity are basically in atrade-off relation, a decrease in responsivity associated with highspeed is inevitable to some extent, however, according to the presentinvention, it is possible to change the responsivity over a wide rangeeven when the absorption layer is decreased in thickness for high speedoperation. However, the operation speed is decreased to some extent byincreasing the device capacitance associated with an increase inphoto-detector length.

FIG. 36 shows a calculation result in InP of extension factor toabsorption layer thickness of effective absorption length relative torefractive index (n₁) of the substance between the device and fiberwhen, as described above, light of wavelength 1.55 μm is incident at amesa angle of 55 degrees. The extension factor can be substantiallyincreased by using a material having a large n₁. In the presentembodiment, polyimide having a refractive index of 1.7 is used, theextension factor in this case provides a 41% increase relative to air,as a result, the above-described increase in responsivity is obtained.It is needless to say that the substance between the device and fibermay be any one which has a good transmissivity to incident light and arelative index of more than 1, selected from organic substances such asvarious polyimides including fluorinated polyimides, various epoxiesincluding epoxy resins, fluorinated epoxies, fluorinated epoxy acrylateresins, acrylics, metamorphic silicone resins, and inorganic substancessuch as chalcogenide glass having a refractive index of more than 2, ora liquid substance such as oil. By appropriately selecting such varioussubstances, it is possible to change the refraction angle at the lightincident facet of the photo-detector, even when using a refraction typesemiconductor photo-detector cut from the same wafer having the samelayer structure and the same mesa angle construction, and adjustment ofresponsivity according to the application is possible.

As to the photo-detector, in the present embodiment, the p-InP layer atthe surface side is formed by crystal growth, however, alternatively, anundoped InP layer may be formed in crystal growth, and the conductiontype of the main part at the surface side be determined by Zn diffusionor an ion implantation method and subsequent anneal. Further, as thesemiconductor photo-detector part, on a semiconductor layer having afirst conduction type, a multilayer structure having a largeschottky-barrier height opposing a schottky electrode which has aschottky-barrier higher than the schottky barrier between thephoto-absorption layer and the schottky electrode may be constructed onthe substrate, between the photo-absorption layer comprising anintrinsic or first conduction type semiconductor layer, a superlatticesemiconductor layer, or a multiple quantum well semiconductor layer andthe schottky electrode, and a semiconductor photo-detector may beconstructed with the semiconductor layer of large schottky barrierheight comprising In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) orIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.

Yet further, the present embodiment is an example using n-InP as thesubstrate, however, a p-InP layer can be used by reversing the above pand n, or a semi-insulating InP substrate can also be used infabrication by considering the leading method of the electrode.

Yet further, a uniform composition bulk is used here as thephoto-absorption layer, however, it is needless to say that aSeparate-absorption-graded-multiplication (SAGM) structure or a Separateabsorption and multiplication superlattice (SAM-SL) structure (used inan avalanche photodiode) or a semiconductor layer of other superlatticestructure or the like may be used.

Yet further, it is needless to say that a material system other thanInGaAsP/InP such as InGaAlAs/InGaAsP or AlGaAs/GaAs system or a materialsystem including a strain may be used.

Yet further, as to the optical waveguide, a single mode optical fiber isused in the present embodiment, however, alternatively, a lensed fiber,a tapered fiber, a silica-based waveguide circuit such as PlanarLightwave Circuit (PLC) or the like may be used.

Yet further, it is needless to say that a hybrid integrated constructionof mounting a photo-detector on PLC may be used.

Yet further, it is needless to say that the waveguide is not only thosebased on various inorganic materials such as silica-based waveguide butmay be those based on various organic materials such as polymerwaveguide or plastic fiber.

The present invention has been described in detail with respect topreferred embodiments, and it will now be apparent from the foregoing tothose skilled in the art that changes and modifications may be madewithout departing from the invention in its broader aspects, and it isthe intention, therefore, in the appended claims to cover all suchchanges and modifications as fall within the true spirit of theinvention.

What is claimed is:
 1. A semiconductor photo-detector, comprising: anintrinsic or a first conduction type semiconductor layer, aphoto-absorption layer comprising a superlattice semiconductor layer ora multiple quantum well semiconductor layer, and a schottky electrodewhich are disposed on a substrate having a top surface and an endsurface meeting at an edge; said photo-absorption layer being spacedfrom said edge of said substrate adjoining said end surface; asemiconductor multilayer structure of large schottky-barrier heighthaving a schottky barrier higher in schottky barrier height than aschottky barrier between said photo-absorption layer and said schottkyelectrode being formed between said photo-absorption layer and saidschottky electrode; and a light incident facet on said end surfaceforming an acute angle with said top surface, wherein incident light isrefracted at said light incident facet and transits saidphoto-absorption layer at an angle with respect to an orthogonal of saidphoto-absorption layer.
 2. The semiconductor photo-detector as claimedin claim 1, wherein said semiconductor layer of large schottky-barrierheight comprises In_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1).
 3. Thesemiconductor photo-detector as claimed in claim 1, wherein saidsemiconductor layer of large schottky-barrier height comprisesIn_(1-x-y)Ga_(x)Al_(y)As (0≦x≦1, 0≦y≦1) and thinIn_(1-u)Ga_(u)As_(1-v)P_(v) (0≦u≦1, 0≦v≦1) disposed thereon.
 4. Thesemiconductor photo-detector as claimed in claim 1, wherein acompositionally graded or step-graded layer from the same composition assaid photo-absorption layer to the same composition as saidsemiconductor layer of large schottky-barrier height is disposed betweensaid photo-absorption layer and said semiconductor layer of largeschottky-barrier height.
 5. A semiconductor photo-detector, comprising:a substrate having a top surface and an end surface meeting at an edge;a photo-absorption part comprising a semiconductor multilayer structureincluding a photo-absorption layer provided on said top surface of saidsubstrate and spaced from said edge; a light incident facet on said endsurface forming an acute angle with said top surface; and a V- orU-shaped groove opposed to said light incident facet, wherein incidentlight from an optical fiber disposed in said groove is refracted at saidlight incident facet and transits said photo-absorption layer at anangle with respect to an orthogonal of said photo-absorption layer. 6.The semiconductor photo-detector as claimed in claim 5, wherein saidlight incident facet and said V- or U-shaped groove are fabricatedsimultaneously by etching.
 7. The semiconductor photo-detector asclaimed in claim 5, wherein said light incident facet and the vicinitythereof are buried in an organic substance.
 8. A semiconductorphoto-detector, comprising: a substrate having a top surface and an endsurface meeting at an edge; a photo-absorption part comprising asemiconductor multilayer structure including a photo-absorption layerprovided on said top surface of said substrate; a light incident faceton said end surface forming an acute angle with said top surface; saidend surface including an abutting surface positioned below and spacedlaterally from said light incident facet and said edge for receiving anoptical waveguide to contribute to precisely positioning said opticalwave guide; and incident light from said optical waveguide beingrefracted at said light incident facet and transiting saidphoto-absorption layer at an angle with respect to an orthogonal of saidphoto-absorption layer when introduced to said light incident fact.
 9. Asemiconductor photo-detector, comprising: a substrate having a topsurface and an end surface meeting at an edge; a photo-absorption partcomprising a semiconductor multilayer structure including aphoto-absorption layer provided on said top surface of said substrate; alight incident facet on said end surface forming an acute angle withsaid top surface; and an upper layer over said photo-absorption layer insaid photo-absorption part being terminated with a substance having asmaller refractive index than a semiconductor layer, wherein incidentlight is refracted at said light incident facet and transits saidphoto-absorption layer at an angle with respect to an orthogonal of saidphoto-absorption layer such that said transit light is totally reflectedby said smaller refractive index substance terminating said upper. 10.The semiconductor photo-detector as claimed in claim 2, wherein acompositionally graded or step-graded layer from the same composition assaid photo-absorption layer to the same composition as saidsemiconductor layer of large schottky-barrier height is disposed betweensaid photo-absorption layer and said semiconductor layer of largeschottky-barrier height.
 11. The semiconductor photo-detector as claimedin claim 3, wherein a compositionally graded or step-graded layer fromthe same composition as said photo-absorption layer to the samecomposition as said semiconductor layer of large schottky-barrier heightis disposed between said photo-absorption layer and said semiconductorlayer of large schottky-barrier height.